Exhibit 10.81

PURCHASE AGREEMENT

As of February 16, 2006

Citigroup Global Markets Inc.

390 Greenwich Street

New York, New York 10013

Attention: Corporate Equity Derivatives

Ladies and Gentlemen:

Amgen Inc., a Delaware corporation (the “Company”), subject to the terms and conditions and in reliance upon the representations and warranties set forth herein, confirms its agreement with Citigroup Global Markets Inc. (the “Dealer”) to purchase from the Dealer 14,188,162 shares (the “Initial Shares”), of the Company’s common stock, $0.0001 par value per share (the “Common Stock”), at a per share price of $81.00 (the “Initial Price”) (subject to adjustment as provided herein). Prior to the close of business on the first Trading Day immediately following the date hereof (the “Settlement Date”), (A) the Company will pay for the Initial Shares by delivering an amount equal to the Aggregate Purchase Price (as hereinafter defined) by wire transfer of immediately available funds to an account designated by the Dealer and (B) the Dealer will deliver the Initial Shares to the Company. The parties understand and agree that the delivery of the Initial Shares by or on behalf of the Dealer upon the payment of the Aggregate Purchase Price by the Company is irrevocable and that as of the Settlement Date the Company shall be the sole beneficial owner of the Initial Shares for all purposes.

The parties to this Agreement agree that the purchases of shares of Common Stock anticipated by this Agreement shall be made pursuant to the requirements of and in conformity with the provisions of Rule 10b5-1 under the Exchange Act (as hereinafter defined), and a plan established by the Company as permitted by Rule 10b5-1 (the “Plan”) described in Annex B hereto.

Section 1. Purchase Price Adjustment.

(a) [Reserved]

(b) For each Trading Day, commencing on the Settlement Date, the Calculation Agent (as hereinafter defined) shall determine the following amounts, as applicable:

(i) The Purchase Price Adjustment (as hereinafter defined) owed to the Dealer by the Company on the Excess Daily Value (as hereinafter defined), if any, for each prior Trading Day;

(ii) The Purchase Price Adjustment owed to the Company by the Dealer on the Deficit Daily Value (as hereinafter defined), if any, for each prior Trading Day;

(iii) The Daily Rebate Value (as hereinafter defined) owed to the Company by the Dealer on a Daily Notional Amount (as hereinafter defined), if any, for each prior Trading Day; and

(iv) The value (which may be positive or negative) equal to the sum of the Purchase Price Adjustment pursuant to clause (ii) above and the Daily Rebate Value pursuant to clause (iii) above minus the Purchase Price Adjustment pursuant to clause (i) above with respect to each day during the Transaction Term (a “Daily Accrual Value”).

(c) On the tenth Trading Day immediately following the last day of the Transaction Term (the “Final Settlement Date”), the Dealer shall pay the Final Settlement Value if the Final Settlement Value is negative or the Company shall pay the Final Settlement Value if the Final Settlement Value is positive.

(d) In the event that the Final Settlement Value is positive, prior to the close of business on the Final Settlement Date, the Company shall cause to be delivered the lesser of (Y) Final Stock Settlement Shares, the value of which is equal to the Final Settlement Value or (Z) the Cap Amount (such lesser amount, the “Positive Final Settlement Value”). If the Company represents to the Dealer that the Company is not in possession of material non-public information or if the Company has terminated the Plan pursuant to its terms, then the Company may, in lieu of the foregoing, elect at its discretion to pay to the Dealer an amount in cash (by wire transfer of immediately available funds) equal to the Positive Final Settlement Value. Such election by the Company to pay cash instead of shares of Common Stock shall be made by the second Trading Day immediately succeeding the notice by the Dealer to the Company that the Final Settlement Value is positive.

If a Stock Settlement Deficiency exists, the Dealer will notify the Company within five (5) Trading Days of the determination of such Stock Settlement Deficiency. Within three (3) Trading Days of such notification, the Company shall deliver to the Dealer shares of Common Stock, the value of which is equal to the Stock Settlement Deficiency Amount (such number of shares being based on the Closing Price of the Common Stock on the third Trading Day immediately succeeding the date of the notification by the Dealer to the Company of the Stock Settlement Deficiency). If the Company delivers shares of Common Stock pursuant to the preceding sentence, the Company shall be obligated to deliver shares of Common Stock to the Dealer, upon notification by the Dealer, until such time as the Dealer has received an amount

from the sale of such shares equal to the Final Settlement Value or until such time as the Company has delivered the amount of shares which is equal to the Cap Amount. If the Company represents to the Dealer that the Company is not in possession of material non-public information or if the Company has terminated the Plan pursuant to its terms, then the Company may, in lieu of the foregoing, elect at its discretion to pay to the Dealer an amount in cash (by wire transfer of immediately available funds) equal to the Stock Settlement Deficiency instead of delivering shares of Common Stock anticipated by the first sentence of this paragraph. Such election by the Company to pay cash instead of shares of Common Stock shall be made by the second Trading Day immediately succeeding the notice by the Dealer to the Company of Stock Settlement Deficiency.

If a Stock Settlement Excess exists, the Dealer will notify the Company within five (5) Trading Days of the determination of such Stock Settlement Excess. Within three (3) Trading Days of such notification, the Dealer shall deliver to the Company the Stock Settlement Excess Amount. If the Company represents to the Dealer that the Company is not in possession of material non-public information or if the Company has terminated the Plan pursuant to its terms, then the Company may, in lieu of the foregoing, elect at its discretion to have the Dealer pay an amount in cash (by wire transfer of immediately available funds) equal to proceeds received by Dealer from the sale of the Stock Settlement Excess Amount instead of delivering shares of Common Stock anticipated by the immediately preceding sentence. Such election by the Company to receive cash instead of shares of Common Stock shall be made by the second Trading Day immediately succeeding the notice by the Dealer to the Company of Stock Settlement Excess.

In the event that the Final Settlement Value is negative, the Dealer shall cause such amount to be delivered to the Company. The Dealer shall satisfy such obligation by delivery to the Company of a number of shares of Common Stock equal to the quotient obtained by dividing the Final Settlement Value by the average per share purchase price paid by the Dealer to acquire (in a commercially reasonable manner) such shares of Common Stock. If the Company represents to the Dealer that the Company is not in possession of material non-public information or if the Company has terminated the Plan pursuant to its terms, then the Company may, in lieu of the foregoing, elect at its discretion to have the Dealer pay an amount in cash (by wire transfer of immediately available funds) equal to the Final Settlement Value instead of delivering shares of Common Stock anticipated by the immediately preceding sentence. Such election to receive cash instead of shares of Common Stock shall be made by second Trading Day immediately succeeding the notice by the Dealer to the Company that the Final Settlement Value is negative.

If the Dealer is unable to purchase a total number of shares of Common Stock equal to the Initial Shares by the deadline established in the definition of “Maturity Date,” then such deadline shall be postponed to a date determined by the Dealer in a written notice to the Company that would enable the Dealer to purchase a total number of shares of Common Stock equal to the Initial Shares. For the purposes of clarity, it is understood and acknowledged by the parties hereto that such postponement shall, among other consequences, extend the Transaction Term and that additional postponements may be required if the Dealer continues to be unable to purchase a total number of shares of Common Stock equal to the Initial Shares by any postponed deadline for the Maturity Date.

The Final Stock Settlement Shares and any other shares of Common Stock made as payment by the Company to the Dealer pursuant to Section 1(d) shall be delivered by the Company in shares of Common Stock the resale of which may be unregistered or registered

under the Securities Act (in the Company’s sole discretion). In the event the Company elects to deliver shares pursuant to Section 1(d) that are intended to be registered under the Securities Act, no later than the Trading Day immediately prior to any delivery, the Company shall have executed and delivered to the Dealer the Registration Rights Agreement. In the event that shares which are not intended to be registered under the Securities Act are delivered to the Dealer pursuant to Section 1(d), the Dealer shall, in consultation with the Company, determine the value of such shares by applying a commercially reasonable discount (which discount shall reflect any costs associated with the delay in resale addressed in the next sentence). If at the time of the delivery and resale of any shares which are not intended to be registered under the Securities Act the Company is unable to represent that the Company is not in possession of material non-public information, then the Dealer shall delay the resale of such shares until such representation may be made.

Section 2. Anti-dilution Adjustments.

(a) Subdivisions and Combination of Common Stock. In the event that the outstanding shares of the Common Stock shall be subdivided or split (including by means of a stock dividend) into a greater number of shares of Common Stock where the effective date of such subdivision or the record date for such split occurs during the Transaction Term, the Initial Shares, the Daily Share Purchase Amount, the Cap Amount and the other share-based terms used herein shall be proportionately increased and the Initial Price shall be deemed to be proportionately decreased. Conversely, in the event that the outstanding shares of Common Stock shall each be combined into a smaller number of shares of Common Stock through a combination of shares of Common Stock or a reverse stock split where the effective date of such combination or the record date for such reverse stock split occurs during the Transaction Term,

the Initial Shares, the Daily Share Purchase Amount, the Cap Amount and the other share-based terms used herein shall be proportionately decreased and the Initial Price shall be proportionately increased. Any adjustment pursuant to this Section 2(a) shall become effective (i) in the case of a subdivision or combination of the Common Stock, on the effective date of such subdivision or combination or (ii) in the case of a stock split or reverse stock split, at the close of business on the record date for such stock split or reverse stock split. Notwithstanding anything to the contrary contained herein, no adjustment shall be made pursuant to this Section 2(a) unless a similar adjustment is required to be made to the number of shares of Common Stock delivered or deliverable to the lender or lenders of Common Stock to the Dealer.

(b) Reclassification, Consolidation, Merger or Sale of Assets. In the event that during the Transaction Term the Company shall enter into any agreement, arrangement or understanding that provides for any recapitalization or reclassification of the Common Stock (other than a change in par value, or from par value to no par value, or from no par value to par value, or as a result of an event specified in Section 2(a)), any consolidation of the Company with, or merger of the Company into, any other person, any merger of another person into the Company (other than a merger which does not result in a reclassification, conversion, exchange or cancellation of outstanding shares of Common Stock), any sale or transfer of all or substantially all of the assets of the Company or any compulsory share exchange and pursuant to any of which the Common Stock is converted into the right to receive other securities, cash or other property (each of the foregoing, an “Extraordinary Transaction”), then the Dealer and the Company shall negotiate in good faith to amend this Agreement to give appropriate effect to the Extraordinary Transaction. In the event that the parties are unable to reach an agreement on the earlier of (i) twenty (20) Trading Days prior to the date, if any, that is specified for the consummation of such

transaction under the governing legal agreements for such transaction and (ii) ten (10) Trading Days after the first public disclosure of the contemplated Extraordinary Transaction (such earlier date, the “Early Termination Date”), (w) the Transaction Term shall be deemed to terminate on the fifth Trading Day after the Early Termination Date, (x) the provisions of Section 3(b)(i) shall be void and of no further force or effect from and after the Early Termination Date, (y) the Final Settlement Date shall be the eighth Trading Day after the Early Termination Date and (z) the Final Settlement Value shall be determined in a commercially reasonable manner by the Calculation Agent in consultation with the Company and the Dealer.

(c) Stock Borrow. In the event the Dealer cannot borrow a sufficient number of shares of Common Stock equal to the remaining number of Initial Shares not repurchased prior to such time at an average cost equal to the Spread or less, the Dealer, may request a change to the Spread to directly compensate for such cost above the Spread.

Section 3. Covenants.

(a) The Company covenants and agrees with the Dealer:

(i) during the Transaction Term, (A) neither the Company nor any of its affiliates shall take any action that would cause the purchases by the Dealer pursuant to Section 3(b)(i) of this Agreement not to comply with the provisions of Rule 10b-18(b)(1) under the Exchange Act as if such provisions applied and (B) the Company will provide the Dealer with all information necessary for Dealer to comply with Rule 10b-18(b)(4) as if such provisions applied;

(ii) during the Transaction Term, to promptly notify the Dealer telephonically (which oral communication shall be promptly confirmed by telecopy to the Dealer) that as a result of an acquisition or other business combination transaction or for any

other reason, the Company determines that the Company will be engaged in a distribution of shares of Common Stock or other securities for which Common Stock is a reference security for purposes of Rule 102 of Regulation M under the Exchange Act and to promptly notify the Dealer by telecopy of the period commencing on the date that is one (1) business day before the commencement of such distribution and ending on the day on which the Company completes the distribution;

(iii) during the Transaction Term, the Company shall not (i) alter its dividend policy that is in effect on the date hereof, (ii) declare an extraordinary dividend or (iii) set an ex-dividend date prior to the Maturity Date; and

(iv) the Company will pay the reasonable and documented fees and expenses of Davis Polk & Wardwell, counsel to the Dealer in connection with this Agreement and the transactions contemplated hereby.

(b) the Dealer covenants and agrees with the Company:

(i) subject to clauses (ii), (iii), (iv) and (v) below, to use its best efforts to purchase, or cause to be purchased, on each Trading Day during the Transaction Term the Daily Share Purchase Amount on the open market at the then market price;

(ii) in connection with bids and purchases pursuant to clause (i) above, the Dealer shall comply, or cause compliance, with the timing and volume provisions of Rule 10b-18(b)(2) and (4) under the Exchange Act as if such provisions applied;

(iii) in connection with bids and purchases pursuant to clause (i) above, the Dealer will effect purchases at a purchase price that does not exceed the highest independent bid or the last independent transaction price, whichever is higher, reported in the

consolidated system at the time such purchases are effected (as those terms are defined in Rule 10b-18 under the Exchange Act);

(iv) not to purchase shares of Common Stock on any Trading Day with respect to which the Dealer reasonably determines in good faith that it is required, in light of legal or regulatory requirements or related policies and procedures reasonably adopted by the Dealer, to refrain from purchasing shares of Common Stock on any such Trading Day. The Dealer shall promptly notify the Company upon exercising its rights pursuant to this clause (iv) and shall subsequently promptly notify the Company on the day the Dealer shall resume purchasing shares of Common Stock pursuant to clause (i) above, it being understood that the Dealer shall not be required to indicate to the Company the reason for the Dealer’s exercise of its rights pursuant to this clause (iv) if the Dealer reasonably determines in good faith that disclosing such reason to the Company may result in a violation of federal or state securities laws or is prohibited by the Dealer’s internal conflicts policies and procedures; and

Section 4. Representations and Warranties.

The Company hereby represents and warrants to the Dealer that as of the date hereof and each Trading Day during the Transaction Term:

(a) the Company has all power and authority to execute this Agreement and enter into the Plan and the transactions contemplated hereby (other than with respect to discretionary actions which, if undertaken by the Company, shall be duly authorized by the Board of Directors of the Company);

(b) this Agreement has been duly authorized, validly executed and delivered by the

Company and constitutes a legal, valid and binding agreement of the Company, enforceable against the Company in accordance with its terms (subject, as to enforcement of remedies, to applicable bankruptcy, reorganization, insolvency, moratorium, fraudulent conveyance or other similar laws affecting the rights of creditors now or hereafter in effect, and to equitable principles that may limit the right to specific enforcement of remedies);

(c) the Company is not entering into this Agreement (i) to create actual or apparent trading activity in the Common Stock (or any security convertible into or exchangeable for Common Stock) or (ii) to facilitate a future distribution of the Common Stock (or any security convertible into or exchangeable for Common Stock) or in connection with a future issuance of securities as part of a plan, in either case with the intention to manipulate the price of the Common Stock (or any security convertible into or exchangeable for Common Stock);

(d) the purchase of the Initial Shares by the Company, the compliance by the Company with all of the provisions of this Agreement and the consummation of the transactions herein contemplated will not result in any violation of the provisions of the Restated Certificate of Incorporation, as amended, or Amended and Restated Bylaws, as amended, of the Company or any statute or any rule or regulation of any court or governmental agency or body having jurisdiction over the Company or any of its properties;

(e) no consent, approval, authorization, order, registration or qualification of or with any court or governmental agency or governmental body having jurisdiction over the Company is required for the purchase of the Initial Shares by the Company, the compliance by the Company with all the terms of this Agreement, or the consummation by the Company of the transactions contemplated by this Agreement, other than the registration of shares of Common Stock pursuant to the Registration Rights Agreement; and

(f) the Company has made its own independent inquiry as to the legal, tax, credit and accounting aspects of the transactions contemplated by this Agreement and any related transactions, and the Company has not relied on the Dealer, the Dealer’s legal counsel or the Dealer’s accounting advisors for legal, tax, credit or accounting advice in connection with the transactions contemplated by this Agreement or any related transactions. The Company agrees and acknowledges that the Dealer and its affiliates may from time to time, not in the capacity of the Company’s agent but in the ordinary course of their business, execute transactions for their own account or the account of customers and hold and deal in securities or options on securities of the Company (including, without limitation, Common Stock) and that the Dealer and its affiliates may continue to conduct such transactions during the Transaction Term.

The Dealer hereby represents and warrants to the Company that:

(a) the Dealer has all power and authority to execute this Agreement and to consummate the transactions contemplated hereby;

(b) this Agreement has been duly authorized, validly executed and delivered by the Dealer and constitutes a legal, valid and binding agreement of the Dealer, enforceable against the Dealer in accordance with its terms (subject, as to enforcement of remedies, to applicable bankruptcy, reorganization, insolvency, moratorium, fraudulent conveyance or other similar laws affecting the rights of creditors now or hereafter in effect, and to equitable principles that may limit the right to specific enforcement of remedies);

(c) the Dealer has made its own independent inquiry as to the legal, tax, credit and accounting aspects of the transactions contemplated by this Agreement and any related transactions, and the Dealer has not relied on the Company or its legal counsel or accounting advisors for legal, tax, credit or accounting advice in connection with the transactions contemplated by this Agreement or any related transactions; and

(d) the Dealer acknowledges that its rights under this Agreement (other than Section 5) do not directly or indirectly give rise to any rights or claims against the Company as a creditor of the Company.

Section 5. Indemnification.

(i) The Company agrees to indemnify the Dealer and its affiliates and their respective directors, officers, employees, agents and controlling persons (the Dealer and each such person being an “Indemnified Party”) from and against any and all losses, claims, damages and liabilities, joint or several, to which such Indemnified Party may become subject (and with respect to which is not duplicative of reimbursements otherwise made pursuant to the terms of this Agreement other than Section 5) under any applicable federal or state law, or otherwise, and related to or arising out of (a) the breach by the Company of any of its representations or warranties contained in this Agreement or the Plan and (b) the breach by the Company of any of its covenants or agreements contained in this Agreement or the Plan, and will reimburse any Indemnified Party for all expenses (including reasonable counsel fees and expenses) in connection with the investigation of, preparation for or defense or settlement of any pending or threatened claim or any action or proceeding arising therefrom, whether or not such Indemnified Party is a party except if such claim, action or proceeding is initiated or brought by or on behalf of the Company. The Company will not be liable under the foregoing indemnification provision to the extent that any loss, claim, damage, liability or expense is found in a final judgment by a court to have resulted directly from willful misconduct or negligence on the part of the Dealer or on the part of any other Indemnified Party.

(ii) If the indemnification provided for in this Agreement is for any reason held unenforceable, the Company agrees to contribute to the aggregate losses, claims, damages and liabilities (including legal or other expenses reasonably incurred in connection with the same) for which such indemnification is held unenforceable as shall be appropriate to reflect (1) the relative fault of the Company on the one hand and the Indemnified Parties on the other hand in connection with the actions or inactions that have resulted in such losses, claims, damages, liabilities and expenses, (2) the relative benefits received by the Company on the one hand and the Dealer on the other hand from the transactions contemplated by this Agreement and (3) any other relevant equitable considerations. Relative fault shall be determined by reference to, among other things, each such party’s relative intent, knowledge, access to information and opportunity to correct or prevent such action or inaction. The Company and the Dealer each agree that it would not be just and equitable if contribution pursuant to this subparagraph (ii) were to be determined by pro rata allocation or by any other method of allocation that does not take account of the equitable considerations referred to above. Notwithstanding the provisions of this Section 5, the Dealer shall not be required to contribute in excess of the amount equal to the excess of (x) the compensation received by the Dealer pursuant to this Agreement over (y) the amount of any damages which the Dealer has otherwise been required to pay by reason of any such action or inaction.

(iii) The Company agrees that without the prior written consent of the Dealer, which consent shall not be unreasonably withheld, it will not settle, compromise or consent to the entry of any judgment in any pending or threatened claim, action or proceeding in respect of which indemnification could be sought under the indemnification provision of this Agreement unless such settlement, compromise or consent includes an unconditional release of each Indemnified Party from all liability arising out of such claim, action or proceeding.

(v) The provisions of this Section 5 shall survive any termination of this Agreement or completion of the transactions contemplated hereby for one (1) year.

(vi) Promptly after receipt by an Indemnified Party of notice of the commencement of any action, such Indemnified Party will, if a claim in respect thereof may be made against the Company under this Section 5, notify the Company in writing of the commencement thereof, but the omission so to notify the Company will not relieve it from any liability which it may have to any Indemnified Party otherwise than under this Section 5 except to the extent that the Company’s rights are materially prejudiced as a result of such delay. In case such notice of any such action shall be so given, the Company shall be entitled to participate at its own expense in the defense, or if it so elects, to assume the defense of such action, in which event such defense shall be conducted by counsel chosen by the Company and reasonably satisfactory to the Indemnified Party or Indemnified Parties who shall be defendant or defendants in such action, and such defendant or defendants shall bear the fees and expenses of any additional counsel retained by them; but if the Company shall elect not to assume the defense of such action, the Company will reimburse such Indemnified Party or Indemnified Parties for the reasonable fees and expenses of any counsel retained by them; provided, however, if the defendants in any such action (including impleaded parties) include both the Indemnified Parties and the Company and counsel for the Company shall have reasonably concluded that there may be a conflict of interest involved in the representation by a single counsel of both the Indemnifying Parties and the Company, the Indemnified Party or Indemnified parties shall have the right to select separate counsel, satisfactory to the Company (it being understood, however, that the Company shall not be liable for the expenses of more than one separate counsel representing the Indemnified Parties who are parties to such action.

Section 6. Certain Definitions.

As used herein the following terms shall have the meanings set forth below:

“Actual Share Purchase Amount” shall mean the actual number of shares of Common Stock purchased by the Dealer pursuant to Section 3(b)(i) of this Agreement on any given Trading Day.

“Actual Share Purchase Value” shall mean, on any given Trading Day, the product of the Actual Share Purchase Amount and the corresponding Settlement Price.

“Aggregate Actual Share Purchase Value” shall mean the amount equal to the aggregate value of all Actual Share Purchase Values, as calculated during the Transaction Term.

“Aggregate Purchase Price” shall mean an amount equal to the Initial Price multiplied by the number of Initial Shares.

“Aggregate Purchase Price Adjustment Value” shall mean the sum (which may be positive, if the Dealer owes the Company value, or negative, if the Company owes the Dealer value) of all Daily Accrual Values for each Trading Day during the Transaction Term.

“Applicable Adjustment Rate” shall mean, for any given Trading Day, an interest rate equal to the Daily Federal Funds Rate.

“Calculation Agent” shall mean the Dealer.

“Cap Amount” shall mean 16,400,000 shares.

“Closing Price” on any day shall mean the last reported sales price regular way of the Common Stock on such day or, in case no such sales price is reported on such day, the average of the reported closing bid and asked prices of the Common Stock, in each case on the NASDAQ, or if not then traded on the NASDAQ, the principal securities exchange or quotation system on which the Common Stock is then listed or admitted to trading, or if not then listed or

admitted to trading on a securities exchange or quotation system, the average of the closing bid and asked prices of the Common Stock in the over-the-counter market on the day in question as reported by the National Quotations Bureau Incorporated, or a similarly generally accepted reporting service, or, if not so available in such manner, as furnished by any NASDAQ member firm selected by the Calculation Agent.

“Daily Effective Rate” shall mean an amount determined by the Calculation Agent equal to the Daily Federal Funds Rate less the Spread.

“Daily Federal Funds Rate” shall mean, with respect to any Trading Day, the rate on such date for United States dollar federal funds as published in H.15(519) under the heading “Federal Funds (Effective)”.

“Daily Notional Amount” shall mean an amount determined by the Calculation Agent equal to the product of (i) the Initial Price and (ii) the amount by which the Initial Shares exceeds the sum of all Actual Share Purchase Amounts which have been executed up to and including the Trading Day preceding the applicable Trading Day.

“Daily Rebate Value” shall mean an amount determined by the Calculation Agent equal to the product of (i) the Daily Effective Rate, (ii) 1/360 and (iii) each corresponding Daily Notional Amount.

“Daily Share Purchase Amount” shall mean an amount to be determined by the Dealer up to the maximum amount of shares of Common Stock permitted to be purchased pursuant to Rule 10b-18 (the “10b-18 Amount”); provided, however, that such amount shall not be less than the lesser of (1) 1,000,0000 shares of Common Stock and (2) the 10b-18 Amount.

“Daily Share Purchase Value” shall mean the product of the Actual Share Purchase Amount and the Initial Price.

“Deficit Daily Value” shall mean, on any given Trading Day, if the Settlement Price is less than the Initial Price, the positive value by which the Daily Share Purchase Value exceeds the Actual Share Purchase Value, but in no event less than zero.

“Excess Daily Value” shall mean, on any given Trading Day, if the Settlement Price is greater than the Initial Price, the positive value by which the Actual Share Purchase Value exceeds the Daily Share Purchase Value, but in no event less than zero.

“Exchange Act” shall mean the Securities Exchange Act of 1934, as amended.

“Final Settlement Value” shall mean (i) the Aggregate Actual Share Purchase Value minus (ii) the sum of (A) the Aggregate Purchase Price plus (B) the Aggregate Purchase Price Adjustment Value.

“Final Stock Settlement Shares” shall mean a number of shares of Common Stock determined by the Calculation Agent (rounded up or down to the nearest whole number) equal to the number of shares of Common Stock that the Company would be required to deliver to the Dealer to satisfy its obligations to the Dealer pursuant to Section 1, based on the Closing Price of the Common Stock as of the Common Stock on the third Trading Day immediately succeeding the date of notification by the Dealer to the Company that the Final Settlement Value is positive.

“Maturity Date” shall mean the date on which the total number of shares of Common Stock purchased by the Dealer pursuant to and for purposes of satisfying the Dealer’s obligation under Section 3(b)(i) of this Agreement (including for purposes of determining the Final Settlement Value) is equal to or greater than the Initial Shares, provided, that such date may not be after March 10, 2006 (which date is subject to postponement pursuant to Section 1(d) hereof).

“NASDAQ” shall mean The NASDAQ Stock Market.

“Purchase Price Adjustment” shall mean adjustment amounts accrued on the Excess

Daily Value or Deficit Daily Value, as applicable, and excluding the Trading Day on which such Excess Daily Value or Deficit Daily Value, as applicable, arises up from the first business day immediately succeeding the Settlement Date to and including the Final Settlement Date at the Applicable Adjustment Rate for such Trading Day.

“Registration Rights Agreement” shall mean the Registration Rights Agreement, substantially in the form of Annex A attached hereto, and with such changes as the parties may mutually agree.

“Securities Act” shall mean the Securities Act of 1933, as amended.

“Settlement Price” shall mean, on any given Trading Day, the weighted average market price per share paid by the Dealer to purchase the Actual Share Purchase Amount.

“Spread” shall mean 20 basis points.

“Stock Settlement Deficiency” shall mean the occurrence of each date, if any, on which the amount received by the Dealer from the sale of the Final Stock Settlement Shares, plus any other shares of Common Stock delivered by the Company to the Dealer pursuant to Section 1(d), is less than the Final Settlement Value.

“Stock Settlement Deficiency Amount” shall mean the amount by which the amount received by the Dealer from the sale of the Final Stock Settlement Shares, plus any other shares of Common Stock delivered by the Company to the Dealer pursuant to Section 1(d), is less than the Final Settlement Value.

“Stock Settlement Excess” shall mean the occurrence of each date, if any, on which the amount received by the Dealer from the sale of the Final Stock Settlement Shares, plus any other shares of Common Stock delivered by the Company to the Dealer pursuant to Section 1(d), equals or exceeds the Final Settlement Value.

“Stock Settlement Excess Amount” shall mean the number of Shares equal to the sum of: (a) the Final Stock Settlement Shares, plus any shares of Common Stock delivered by the Company to the Dealer pursuant to Section 1(d) (together, the “Delivered Shares”), remaining when a Stock Settlement Excess occurs and (b) a number of shares of Common Stock purchased by the Dealer (in a commercially reasonable manner) with any cash proceeds received by the Dealer from the sale of the Delivered Shares in excess of the Final Settlement Value.

“Trading Day” shall mean any day on which the Common Stock is traded on NASDAQ, or, if not then traded on the NASDAQ, the principal securities exchange or quotation system on which such securities are then traded or, if not then traded on a securities exchange or quotation system, in the over-the-counter market.

“Transaction Term” shall mean the period commencing on the Settlement Date and terminating on, and including, the Maturity Date.

Section 7. Miscellaneous.

(a) Severability. If any term, provision, covenant or restriction of this Agreement is held by a court of competent jurisdiction to be invalid, void or unenforceable, the remainder of the terms, provisions, covenants and obligations set forth herein shall remain in full force and effect and shall in no way be affected, impaired or invalidated.

(b) Assignment. Neither the rights under this Agreement nor the obligations created by this Agreement shall be assignable or delegable, in whole or in part, by either party herein without the prior written consent of the other, and any attempt to assign or delegate any rights or obligations arising under this Agreement without such consent shall be void.

(c) Waivers, etc. No failure or delay on the part of either party in exercising any power or right hereunder shall operate as a waiver thereof, nor shall any single or partial exercise of any

such right or power, or any abandonment or discontinuance of steps to enforce such a right or power, preclude any other or further exercise thereof or the exercise of any other right or power. No amendment, modification or waiver of any provision of this Agreement nor consent to any departure by either party therefrom shall in any event be effective unless the same shall be in writing, and, in the case of a waiver or consent, shall be effective only in the specific instance and for the purpose for which given.

(d) Beneficiaries. This Agreement shall be binding upon, and inure solely to the benefit of, the Company and the Dealer and no other person shall acquire any rights hereunder. Without limiting the generality of the foregoing, the Dealer’s obligations under Section 3(b)(i) are solely for the benefit of the Company and not the holders of any of the Company’s securities.

(e) Changes of Law. If, due to any change in applicable law or regulations or the interpretation thereof by any court of law or other body having jurisdiction subsequent to the date of this Agreement, performance of any provision of this Agreement or any transaction contemplated hereby shall become impracticable or impossible, the parties hereto shall use their best efforts to find and employ an alternative means to achieve the same or substantially the same result as that contemplated by such provision.

(f) Confidentiality. Subject (i) to any contrary requirement of law or applicable regulator, (ii) to the right of each party to enforce its rights hereunder in any legal action and (iii) in the case of the Company, to the determination by its counsel that disclosure is appropriate or necessary, each party shall keep strictly confidential and shall cause its employees and agents to keep strictly confidential the terms of this Agreement and any information of or concerning the other party which it or any of its agents or employees may acquire pursuant to, or in the course of performing its obligations under, any provision of this Agreement. The Dealer hereby consents

to the issuance of a press release by the Company announcing its entry into this Agreement and the filing with the Securities and Exchange Commission of such information relating thereto as required under the Exchange Act (in each case in the Company’s sole discretion).

Notwithstanding any provision in this Agreement, in connection with Section 1.6011-4 of the Treasury Regulations, the parties hereby agree that each party (and each employee, representative, or other agent of such party) may disclose to any and all persons, without limitation of any kind, the U.S. tax treatment and U.S. tax structure of the transactions contemplated hereby and all materials of any kind (including opinions or other tax analyses) that are provided to such party relating to such U.S. tax treatment and U.S. tax structure, other than any information for which nondisclosure is reasonably necessary in order to comply with applicable securities laws.

(g) Expenses. The Company will pay or cause to be paid all expenses incident to the performance of its obligations under this Agreement, including the fees and disbursements of the Company’s counsel and accountants and other experts. The Dealer will pay its own expenses incident to the performance of its obligations under this Agreement.

(h) Headings. Descriptive headings herein are for convenience only and shall not control or affect the meaning or construction of any provision of this Agreement.

(i) Counterparts. This Agreement may be executed by the parties hereto in counterparts, and each such executed counterpart shall be, and shall be deemed to be, an original instrument and all such counterparts, taken together, shall constitute one and the same instrument.

(j) Notices. All notices, consents, requests, instructions, approvals and other communications provided for herein shall be validly given, made or served if in writing and delivered personally, by telegram, by telecopy or sent by overnight courier, postage prepaid:

if to the Dealer:

Citigroup Global Markets Inc.

390 Greenwich Street

New York, New York 10013

Attention: Corporate Equity Derivatives

and, in connection with any notices
pursuant to Section 3(a)(ii) by
telephone and facsimile to:

Telephone: 212-723-7357

Facsimile: 212-723-8328

if to the Company:

Amgen Inc.

One Amgen Center Drive

Thousand Oaks, California 91320-1799

Facsimile: 805-499-8011

Attention: Corporate Secretary

with a copy to:

Latham & Watkins LLP

633 West Fifth Street, Suite 4000

Los Angeles, California 90071

Facsimile: 213-891-8763

Attention: Gregory P. Rodgers

or to such other address as any party may, from time to time, designate in a written notice given in a like manner. Notice given by telegram or telecopy shall be deemed delivered when evidence of the transmission is received by the sender and shall be confirmed in writing by overnight courier, postage prepaid. Notice given by overnight courier as set out above shall be deemed delivered the business day after the date the same is mailed.

(k) Governing Law. This Agreement shall be governed by and construed and enforced in accordance with the laws of the State of New York without reference to conflict of law principles.

(l) Cap Amount. For the avoidance of doubt, notwithstanding anything herein to the contrary, in no event shall the Company be obligated to issue or deliver to the Dealer shares of Common Stock pursuant to this Agreement in excess of the Cap Amount (as such amount may be adjusted from time to time pursuant to Section 2).

(m) Waiver of Set-Off. Each of the Dealer and the Company waives any and all rights it may have to set-off under this Agreement, whether arising under any agreement, applicable law or otherwise.

If the foregoing is in accordance with our understanding of our agreement, please sign and return to us the enclosed duplicate hereof, whereupon this letter and your acceptance shall represent a binding Agreement between the Company and you.

		Very truly yours,

		AMGEN INC.


		By
			Name:
			Title:


Accepted as of the date
first written above.

CITIGROUP GLOBAL MARKETS INC.


By
	Name:
	Title: