<PAGE>

                                                                   EXHIBIT 10.24


October 11 , 1999


Dr. N. Kirby Alton
815 Country Valley Road
Thousand Oaks, CA  91362

Re:  Agreement Regarding Part-Time Special Assignment Position
     ---------------------------------------------------------

Dear Kirby:

On behalf of Amgen Inc. ("Amgen"), I am pleased to confirm in this letter
agreement (the "Agreement") the terms and conditions under which you will
continue to be employed by Amgen from and after the date upon which you cease to
serve as Amgen's Senior Vice President of Development, which we currently
contemplate will occur on October 20, 1999 (the "Effective Date").  You will
remain in your current position and receive all compensation and benefits of
that position between now and the Effective Date.  This Agreement also provides
for the termination of your employment with Amgen on or before October 19, 2002,
as set forth below.

1.   POSITION AND DUTIES
     -------------------

     On the Effective Date, you will cease to be a regular full-time employee of
     Amgen and will resign from all offices you hold in Amgen and its
     subsidiaries, but you will continue to be employed by Amgen as an employee
     in a part-time special assignment position, at grade level 37, with the
     title of Special Advisor, Development (in connection with resigning your
     offices, you agree to execute and return to Amgen with this Agreement a
     signed original resignation letter (the "Resignation Letter") on your Amgen
     letterhead in the form provided in Appendix A to this Agreement.  Appendix
     A is hereby incorporated into and made part of the Agreement by reference).
     As Special Advisor, Development, it is anticipated that you will work with
     George Morstyn on Amgen's Product Development efforts.  As we have
     discussed, George would like you to assist him in monitoring regulatory
     developments in the Pharmaceutical industry as they relate to Amgen's
     current products, products which Amgen is in the process of developing and
     potential future products, including those which Amgen may acquire by
     corporate or other acquisitions.  You will provide to George from time to
     time, upon his reasonable request, written or oral reports and/or copies of
     other written materials with regard to the foregoing.

     Also as we have discussed, the position of Special Advisor, Development is
     a part-time special assignment position in which you will be expected to
     work a minimum of ten (10) hours per month and no more than twenty (20)
     hours per month.  The times and places where this work will be performed
     will be at your choosing.  You will maintain a log showing the time you
     have spent performing the foregoing services and this log shall be deemed
     conclusive evidence of the time spent.
<PAGE>

     We have agreed that your part-time special assignment will continue until
     October 19, 2002, subject to extension as you and Amgen may agree in
     writing or to earlier termination by you or Amgen as set forth in Paragraph
     8 of this Agreement.  As long as you are employed by Amgen, you will
     continue to be subject to Amgen's policies and procedures, including but
     not limited to those relating to the non-disclosure of proprietary and
     confidential information and you will continue to be subject to the Amgen
     Inc. Proprietary Information and Inventions Agreement, executed by you on
     or about January 15, 1982 (the "Proprietary Agreement") (which also
     contains obligations that survive the termination of your employment with
     Amgen).

2.   COMPENSATION AND BENEFITS
     -------------------------

     Following is a brief description of the compensation and benefits you will
     receive under this Agreement during your part-time special assignment.  The
     terms and conditions of all of your benefits are subject to the terms and
     conditions of each of the applicable plans, policies or arrangements, as
     they may be amended or terminated by Amgen from time to time.

     2.1  Compensation:  Your compensation will be $72,250 per month, subject to
          ------------
          applicable income tax and employment tax withholding requirements.  In
          addition, Amgen will reimburse you for any reasonable business
          expenses you incur in performing your duties, subject to Amgen's
          standard employee expense reimbursement policies.

     2.2  Administrative Support:  Amgen will provide you with an office and
          ----------------------
          secretarial assistance at our Thousand Oaks headquarters.  You will
          also have access to the services of Amgen's travel department.

     2.3  Management Incentive Plan:  You will not be eligible to participate in
          -------------------------
          Amgen's Management Incentive Plan (the "MIP") for the calendar year
          1999 or for any year after 1999.

     2.4  Special Bonus for 1999 Calendar Year:  As part of the transition to
          ------------------------------------
          the part-time special assignment position, you will be entitled to a
          special bonus in the amount that is the greater of: (1) the MIP
          payment that you would have received for the calendar year 1999 based
          on what would have been your MIP rating for the 1999 calendar year, if
          you had been eligible for that MIP payment and if you had been deemed
          to have been a regular full-time employee for the entire year for
          purposes of the MIP or (2) the MIP payment you received for the 1998
          calendar year.  This special bonus will be paid to you at the same
          time that MIP distributions are made to participants in the MIP in
          2000.

     2.5  Employee Stock Purchase Plan:  You will be eligible to continue to
          ----------------------------
          participate in Amgen's Employee Stock Purchase Plan (the "ESPP") until
          the end of the current purchase period (March 31, 2000).  However, due
          to the fact that you will be working less than twenty (20) hours per
          week, you will not be eligible to participate in the ESPP after the
          current purchase period.

                                       2
<PAGE>

     2.6  Supplemental Retirement Plan:  You will be eligible to participate in
          ----------------------------
          Amgen's Supplemental Retirement Plan (the "SRP") if you continue to
          meet the eligibility requirements of the SRP.  Notwithstanding the
          foregoing, Amgen acknowledges and agrees that your inability to
          participate in the Amgen Retirement and Savings Plan (the "401(k)
          Plan") shall not make you ineligible to participate in the SRP.

     2.7  Retirement and Savings Plan:  Pursuant to Section 3.3 of the 401(k)
          ---------------------------
          Plan, employees that are eligible to participate in the 401(k) Plan
          are those that are classified as "regular full-time" or "regular part-
          time" employees.  By signing below, you expressly acknowledge and
          agree that Amgen is not classifying you as a regular full-time or
          regular part-time employee and therefore, as of the Effective Date,
          you will not be eligible to make contributions or to have
          contributions made on your behalf to the 401(k) Plan.  This letter
          qualifies as an agreement pursuant to Section 3.3(c)(2) of the 401(k)
          Plan.  You will, however, be able to maintain your 401(k) account in
          the Amgen plan to the extent allowed by law.

     2.8  Change of Control Severance Plan:  You will continue to be eligible to
          --------------------------------
          participate in the Amgen Inc. Change of Control Severance Plan (the
          "CIC Plan").  However, on the Effective Date you will cease to be a
          Group I Participant and will become a Group II Participant in the CIC
          Plan by virtue of your ceasing to be a member of Amgen's Operating
          Committee. Notwithstanding the foregoing, in the event that the
          aggregate benefits provided for in this Agreement are greater than
          those provided in the CIC Plan upon a termination of employment for
          which you would be eligible to receive benefits under the terms and
          conditions of the CIC Plan, this Agreement, rather than the CIC Plan
          shall govern and control your rights upon a termination of employment;
          provided, that, in such event, and if applicable, you shall also
          receive the 280G tax gross-up benefit provided in Section 4.1(G) of
          the CIC Plan.

     2.9  Stock Options:
          -------------

          2.9.1  No New Grants:  As an employee in a part-time special
                 -------------
                 assignment position, you will not be eligible to receive
                 additional stock option grants after the Effective Date.

          2.9.2  Accelerated Vesting:  Amgen shall take the necessary action to
                 -------------------
                 accelerate the vesting of the following options from previous
                 grants (and no others, unless an acceleration takes place
                 pursuant to Paragraph 8 of this Agreement) so that the
                 following options will be immediately and fully vested as of
                 the Effective Date (provided that the Agreement shall be
                 effective as provided in Subparagraph 1.2(e) of Appendix B to
                 this Agreement by October 20, 1999) to the extent that they
                 have not previously vested:

                                       3
<PAGE>

<TABLE>
<CAPTION>

No. of Option Shares       Grant No.   Originally Scheduled
------------------------   ---------   --------------------
                                         Expiration Date
                                       --------------------
<S>                           <C>            <C>

                 3,052         933511          7/1/03
                 1,642         945177          7/1/04
                18,948         938853          7/1/03
                 8,013         945178          7/1/03
                 6,372         945178          7/1/04
</TABLE>

                 Amgen shall provide you with a copy of the resolutions taking
                 the action described in this Subparagraph 2.9.2.

          2.9.3  Vesting During Special Assignment:  To the extent that you
                 ---------------------------------
                 continue in your part-time special assignment, you will be
                 eligible to continue to vest in all unvested options that have
                 previously been granted to you by Amgen on the dates and in the
                 manner provided in your stock option grant agreements and
                 applicable stock option plans, except as provided in
                 Subparagraph 2.9.2. No stock options will vest following the
                 Termination Date as defined in Paragraph 8 of this Agreement.

          2.9.4  Cooperation To Restructure:  As we have discussed, it is our
                 --------------------------
                 intention that your ability to continue to vest in and exercise
                 options while in your part-time special assignment position
                 will not result in any additional compensation charges to Amgen
                 in accordance with U.S. generally accepted accounting
                 principles. Accordingly, if at any time Amgen determines that
                 it is reasonably likely that Amgen will incur a compensation
                 charge as a result of your vesting or exercising options in
                 your part-time special assignment position then you agree that
                 you will use your reasonable best efforts to cooperate with
                 Amgen to restructure this Agreement and your position as Amgen
                 reasonably determines is necessary for you to continue to be
                 able to vest and exercise your options without creating a
                 compensation charge to Amgen in accordance with U.S. generally
                 accepted accounting principles and without causing you to lose
                 any of the benefits of this Agreement. However, if no such
                 accommodation is possible without materially affecting your
                 rights hereunder, your rights as set forth herein shall not be
                 modified or altered in any fashion which would have any adverse
                 effect on you.

          2.9.5  No Amendment to Stock Option Grant Agreements or Stock Option
                 -------------------------------------------------------------
                 Plans:  Nothing in this Agreement shall be deemed to alter,
                 -----
                 amend, or otherwise modify the terms of your stock option grant
                 agreements or the terms of the applicable stock option plans.

     2.10 Medical, Dental, and Vision Insurance and COBRA:  Your medical,
          -----------------------------------------------
          dental, and vision insurance coverage will terminate on the Effective
          Date.  If after the Effective Date, you or your eligible dependents
          should elect to continue coverage under

                                       4
<PAGE>

          Amgen's group health plan(s) under the Consolidated Omnibus Budget
          Reconciliation Act ("COBRA") continuation rights, and you or your
          eligible dependents timely take the required steps to initiate such
          coverage, then Amgen will pay the cost of COBRA coverage for you and
          your eligible dependents until the earlier of April 19, 2001, or until
          you and/or your eligible dependents no longer qualify for COBRA
          continuation rights or, in the case of your dependents, the date on
          which such dependents cease to be eligible dependents under Amgen's
          group health plan(s), whichever occurs first. If you and/or your
          eligible dependents qualify for COBRA benefits on or after April 19,
          2001, then you and/or your eligible dependents will have the option of
          continuing coverage under Amgen's group health plan(s), under COBRA
          and at Amgen's expense until October 19, 2002 and at your own expense
          thereafter. If you obtain health insurance coverage for you and/or
          your COBRA eligible dependents for the period between April 19, 2001
          and the Termination Date as defined in Paragraph 8 of this Agreement,
          then Amgen will reimburse you for the full cost of such insurance
          premiums. To receive reimbursement, submit copies of the health
          insurance premium invoices and other applicable information on a
          monthly basis to Amgen. For a complete description of the rights and
          responsibilities you and your eligible dependents have under COBRA,
          you must refer to the COBRA documents that will be sent to you by
          Amgen or its designee under separate cover.

     2.11 Basic Life Insurance:  Your Basic Life Insurance coverage will
          --------------------
          terminate on the Effective Date.  If you are interested in converting
          this insurance to an individual policy, please contact Jean Ellis at
          Aetna (860) 273-7252 within thirty (30) days after the Effective Date.

     2.12 Long-Term Disability Insurance:  Your Long-Term Disability Plan
          ------------------------------
          coverage will terminate on the Effective Date and there is no
          conversion policy or plan available for this coverage.

     2.13 Other Benefits:  As an employee in a part-time special assignment
          --------------
          position, you will not be eligible to participate in the following
          Amgen benefit plans and programs as well as any other benefits not
          specifically listed in this letter:  Dependent Care Assistance
          Program; Medical Flexible Spending Account; Voluntary and Dependent
          Life Insurance Coverage; Accidental Death and Dismemberment benefit;
          use of Amgen Fitness Center facilities; use of Amgen Child Care Center
          facilities; personal illness; vacation/optional holiday pay; family
          illness/personal time; bereavement leave or holidays.  Your accrued
          and unused vacation hours and optional holiday pay will be paid to you
          on the next regularly scheduled payroll date following the Effective
          Date.

3.   TRANSFER OF COMPANY PROPERTY
     ------------------------------

     Except as provided in the remainder of this Subparagraph, you promise that
     on or before the Termination Date, as defined in Paragraph 8 of this
     Agreement, you will return to Amgen all files, memoranda, documents,
     records, copies of the foregoing, credit cards, keys, and

                                       5
<PAGE>

     any other Amgen property in your possession or under your control. As an
     employee in a part-time special assignment position, you will continue to
     have access to and use of those two telecopier machines and two laptop
     computers that Amgen previously provided to you. As of the termination of
     your employment with Amgen, you will be entitled to retain the equipment
     referenced in the preceding sentence provided that you take the steps
     necessary to ensure that all of Amgen's proprietary information is deleted
     from the two laptop computers by Amgen's computer services department as of
     the Termination Date as defined in Paragraph 8 of this Agreement.


4.   OFFICERS AND DIRECTORS INSURANCE
     --------------------------------

     During your part-time special assignment and for four (4) years following
     the Termination Date, you will be covered by such officers and directors
     insurance coverage that Amgen provides to its senior executive officers at
     your salary grade level during that time period.  In addition, Amgen shall
     indemnify and hold you harmless both during and after the entire term of
     your employment (including your service hereunder) to the fullest extent
     permitted by law with regards to actions or inactions in relation to your
     duties performed at Amgen, both before and after the date of this
     Agreement.  Furthermore, you will be entitled to reimbursement of expenses
     incurred in accordance with your rights under California Labor Code Section
     2802.

5.   LEGAL FEE AND FINANCIAL/TAX CONSULTING REIMBURSEMENT
     ----------------------------------------------------

     Amgen will reimburse you for the legal expenses reasonably incurred by you
     in connection with the review of this Agreement up to a maximum amount of
     $20,000.  Amgen will also reimburse you for financial and/or tax counseling
     expenses that you reasonably incur, up to a maximum amount of $3,000 per
     year, for each year of this Agreement.

6.   REFERENCE
     ---------

     Amgen will provide you with a positive written factual reference.  I should
     be listed as your work reference.  You agree to confer with me on the form
     and nature of the reference to be provided to prospective employers and
     other third parties concerning the work that you have performed at Amgen.
     If, by sixty (60) days after the Effective Date, you are unable to reach
     agreement with me on the written reference to be provided, then Amgen's
     only obligation will be to respond to inquiries by confirming to
     prospective employers or other third parties the dates of your  employment
     at Amgen and the last position you held as an Amgen employee.

7.   RELOCATION
     ----------

     If you decide to relocate outside of the fifty (50) mile radius of your
     Residence (as defined below) during the period of your part-time special
     assignment or immediately at the termination thereof for any reason other
     than for a Stated Reason, as defined below, and sell your current, local,
     primary residence located in North Ranch, California (the "Residence")

                                       6
<PAGE>

     so that the sale escrow closes no later than October 19, 2002, then Amgen
     will provide you with the following:

     7.1  If your new employer, if any, provides for part of the following
          expenses, then Amgen would pay normal and customary amounts beyond
          those which such new employer paid, up to the amounts that Amgen would
          normally pay, as of the date your employment with Amgen terminated, to
          newly hired Amgen employees in your job:  normal and customary costs
          for the packing, shipping, delivery, storage (for up to ninety (90)
          days) and unpacking of your common household goods and furnishings.

     7.2  If you shall sell your Residence so that the close of escrow on the
          sale occurs prior to October 19, 2002, then in such event, Amgen will
          reimburse you for those normal and non-recurring customary sales costs
          associated with the sale of such residence, subject to the following
          terms and conditions:

          7.2.1  Amgen's obligation will be limited to that amount which, as of
                 the day immediately prior to the date of this Agreement, Amgen
                 would pay to reimburse other employees of your then salary
                 grade level;

          7.2.2  to the extent that your new employer, if any, reimburses you
                 for, or pays any of, such non-recurring customary sales costs,
                 then Amgen will only reimburse you for that portion of the non-
                 recurring customary sales costs that exceed the amount paid for
                 by such new employer; and

          7.2.3  you provide all documentation requested by Amgen in connection
                 with this Subparagraph 7.2, upon the request of Amgen.

     7.3  If you meet the above conditions and so elect, Amgen will grant you
          the opportunity to place your Residence in the "Amgen Marketing
          Assistance and Homesale Program" (the "Program").  For a description
          of the Program, please contact Christine Swinburne of the Amgen Human
          Resources Department.  In order to participate in the Program, you
          must notify Ms. Swinburne in writing, of your election to participate
          in the Program no later than April 19, 2002, in order to complete the
          home sale process by October 19, 2002.  In order for Amgen to provide
          you with the assistance provided for in this Subparagraph 7.3 in
          connection with the sale of your Residence, you must give Amgen
          control over the disposition of the property, must provide such
          documentation as Amgen may request and must cooperate with Amgen in
          the sale of the Residence.

8.   EARLY TERMINATION OF SPECIAL ASSIGNMENT
     ---------------------------------------

     We have agreed that you will continue in your part-time special assignment
     position until October 19, 2002, at which time your employment with Amgen
     will terminate, provided however, that Amgen may terminate your

                                       7
<PAGE>

    employment prior to October 19, 2002 in the event that a Stated Reason (as
    defined below) occurs, and you may terminate your employment prior to
    October 19, 2002 upon thirty (30) days prior written notice to Amgen in the
    event of a Covered Breach (as defined below) or otherwise.

    For purposes of this Paragraph 8, a "Stated Reason" means (i) your
    conviction of a felony, or (ii) the engaging by you in conduct that
    constitutes willful gross neglect or willful gross misconduct in carrying
    out your duties set forth in Paragraph 1 of this Agreement, resulting, in
    either case, in material economic harm to Amgen, unless you believed in good
    faith that such conduct was in, or not contrary to, the best interests of
    Amgen. For purposes hereof, no act, or failure to act, on your part shall be
    deemed "willful" unless done, or omitted to be done, by you not in good
    faith.

    For purposes of this Paragraph 8, a "Covered Breach" means a breach by Amgen
    of its obligations under this Agreement in the following manner only (i) any
    reduction in your salary or benefits provided for in this Agreement or (ii)
    the assignment of duties to you that are inconsistent with, or greater in
    scope than, those set forth in Paragraph 1 of this Agreement or (iii) a
    reduction in your title or position or the requirement that you work with
    anyone other than George Morstyn or a mutually agreed successor to George
    Morstyn or (iv) a failure by Amgen to have any successor expressly assume
    this Agreement in accordance with Paragraph 17 of this Agreement. In order
    for an event described in the preceding sentence to qualify as a Covered
    Breach, you must give written notice of the event to Amgen and Amgen must
    fail to cure the event within 30 days of receipt of that written notice.

    In the event your employment is terminated by Amgen for a Stated Reason or
    if you terminate your employment for any reason other than a Covered Breach
    then your payments and benefits from Amgen under this Agreement, including
    but not limited to the vesting of your stock options, will cease as of the
    effective date of the termination of your employment.

    In the event your employment is terminated by Amgen not for a Stated Reason
    or if you terminate your employment for a Covered Breach, then (i) you shall
    be paid in a cash lump-sum all of the remaining payments due to you under
    this Agreement from the date of your termination through October 19, 2002,
    (ii) you shall continue to be provided the benefits set forth in Paragraph
    2.10 of this Agreement through October 19, 2002 and (iii) Amgen shall take
    the necessary action to accelerate the vesting of all of your outstanding
    and then unvested stock options so that they shall vest and become
    immediately exercisable in full as of the Termination Date; such stock
    options, as so accelerated shall be exercisable as provided in your stock
    option grant agreements and applicable stock option plans. Amgen shall
    provide you with a copy of the resolutions taking the action described in
    clause (iii) of the preceding sentence.

    The date of the termination of your employment for any of the foregoing
    reasons, or upon your death, is hereinafter referred to as the "Termination
    Date."

                                       8
<PAGE>

9.   DEATH
     -----

     In the event of the termination of your employment hereunder by reason of
     your death prior to October 19, 2002, all of the remaining payments
     pursuant to Paragraph 2.1 of this Agreement will be payable to the
     beneficiary or beneficiaries that you designate in writing to Amgen.  Your
     other remaining benefits will be treated according to their specific terms
     concerning such death.  For purposes of Paragraph 10(a) of the Amgen Inc.
     Amended and Restated 1991 Equity Incentive Plan, your employment with Amgen
     shall be deemed to have commenced in 1982.

10.  RELEASE
     -------

     In exchange for consideration provided to you under this Agreement, you
     hereby agree to execute and be bound by the General Release attached hereto
     as Appendix B (the "General Release") and to return the executed Agreement,
     together with the executed General Release, to me on or before October 12,
     1999.  The General Release is hereby incorporated into and made part of the
     Agreement by this reference.

11.  INTERPRETATION
     --------------

     This Agreement, Resignation Letter attached hereto as Appendix A, and the
     General Release attached hereto as Appendix B shall be construed as a whole
     according to their fair meaning, and not strictly for or against any of the
     parties.  Unless the context indicates otherwise, the term "or" shall be
     deemed to include the term "and" and the singular or plural number shall be
     deemed to include the other.  Paragraph headings used in this Agreement and
     the General Release are intended solely for convenience of reference and
     shall not be used in the interpretation of any of this Agreement or the
     General Release.

12.  NOTICES
     -------

     For the purposes of this Agreement, notices, demands and all other
     communications provided for in this Agreement shall be in writing and shall
     be deemed to have been duly given when delivered either personally or by
     United States certified or registered mail, return receipt requested,
     postage prepaid, addressed, if to you, to the last address on file with
     Amgen and if to Amgen, to its executive offices or to such other address as
     any party may have furnished to the others in writing in accordance
     herewith, except that notices of change of address shall be effective only
     upon receipt.

13.  LEGAL FEES; ARBITRATION
     -----------------------

     13.1  Agreement to Arbitrate:  Any dispute (an "Arbitrable Dispute")
           ----------------------
          arising between the parties, including but not limited to those
          concerning the formation, validity, interpretation, effect, or alleged
          violations of this Agreement or the General Release, must be submitted
          to binding arbitration for resolution in Los Angeles, California in
          accordance with the rules and procedures of the Employment Dispute
          Resolution

                                       9
<PAGE>

          Rules of the American Arbitration Association then in effect. The
          decision of the arbitrator shall be final and binding on both parties,
          and any court of competent jurisdiction may enter judgment upon the
          award. Amgen shall pay all expenses relating to such arbitration,
          including, but not limited to, your legal fees. Except for an action
          taken outside of arbitration pursuant to Subparagraph 13.3 of this
          Agreement, should either party pursue any other legal or
          administrative action against the other, the responding party shall be
          entitled to the return of any payments that party made under the
          Agreement and shall be entitled to recover all costs, expenses and
          attorneys' fees the responding party incurs as a result of such
          action. The arbitrator may not modify or change this Agreement or the
          General Release in any way.

     13.2 Exclusive Remedy:  Arbitration in this manner shall be the exclusive
          ----------------
          remedy for any Arbitrable Dispute.  The arbitrator's decision or award
          shall be fully enforceable and subject to an entry of judgment by a
          court of competent jurisdiction.  Except for an action taken outside
          of arbitration pursuant to Subparagraph 13.3 of this Agreement, should
          you or Amgen, without the consent of the other party, attempt to
          resolve an Arbitrable Dispute by any method other than arbitration
          pursuant to this Paragraph 13, the responding party shall be entitled
          to recover from the initiating party all damages, expenses and
          attorneys' fees incurred as a result.

     13.3 Sole Exception:  Notwithstanding the foregoing, a dispute relating to
          --------------
          the alleged use or disclosure of information which is prohibited by
          the Proprietary Agreement, and/or the criticism, denigration or
          disparagement of Amgen, any other Amgen Releasee, as defined in
          Subparagraph 1.1 of the General Release, or any of Amgen's products,
          processes, experiments, policies, practices, standards of business
          conduct, or areas or techniques of research may be resolved through a
          means other than arbitration, at Amgen's sole option.

14.  GOVERNING LAW
     -------------

     This Agreement is governed by, and is to be construed and enforced in
     accordance with, the laws of the State of California, without regard to
     principles of conflicts of laws.

15.  TAXES
     -----

     You acknowledge and agree that all payments made pursuant to this Agreement
     shall be made less applicable tax withholdings and/or other withholdings as
     required by law.  You acknowledge and agree that you, and not Amgen, shall
     be solely responsible for any taxes imposed upon you as a result of the
     payments and benefits you receive under the Agreement with the sole
     exception of the potential 280G tax gross-up as provided in Subparagraph
     2.8 of this Agreement.

                                       10
<PAGE>

16.  MITIGATION
     ----------

     You shall not be required to mitigate amounts payable under this Agreement
     by seeking other employment or otherwise, and there shall be no offset
     against amounts due you under this Agreement on account of subsequent
     employment.  Additionally, amounts owed to you under this Agreement shall
     not be offset by any claims Amgen may have against you and Amgen's
     obligation to make the payments provided for in this Agreement and
     otherwise to perform its obligations hereunder, shall not be affected by
     any other circumstances, including, without limitation, any counterclaim,
     recoupment, defense or other right which Amgen may have against you or
     others.

17.  SUCCESSORS; BINDING AGREEMENT
     -----------------------------

     17.1 Amgen's Successors:  No rights or obligations of Amgen under this
          ------------------
          Agreement may be assigned or transferred except that Amgen will
          require any successor (whether direct or indirect, by purchase,
          merger, consolidation or otherwise) to all or substantially all of the
          business and/or assets of Amgen to expressly assume and agree to
          perform this Agreement in the same manner and to the same extent that
          Amgen would be required to perform it if no such succession had taken
          place.  As used in this Agreement, "Amgen" shall mean Amgen as herein
          before defined and any successor to its business and/or assets (by
          merger, purchase or otherwise) which executes and delivers the
          agreement provided for in this Paragraph 17 or which otherwise becomes
          bound by all the terms and provisions of this Agreement by operation
          of law.

     17.2 Your Successors:  No rights or obligations of you under this
          ---------------
          Agreement may be assigned or transferred by you other than your rights
          to payments or benefits hereunder, which may be transferred only by
          will or the laws of descent and distribution.  Upon your death, this
          Agreement and all rights of you hereunder shall inure to the benefit
          of and be enforceable by your beneficiary or beneficiaries, personal
          or legal representatives, or estate, to the extent any such person
          succeeds to your interests under this Agreement.  You shall be
          entitled to select and change a beneficiary or beneficiaries to
          receive any benefit or compensation payable hereunder following your
          death by giving Amgen written notice thereof.  In the event of your
          death or a judicial determination of your incompetence, reference in
          this Agreement to you shall be deemed, where appropriate, to refer to
          your beneficiary(ies), estate or other legal representative(s).  If
          your should die following your Termination Date while any amounts
          would still be payable to you hereunder if you had continued to live,
          all such amounts unless otherwise provided herein shall be paid in
          accordance with the terms of this Agreement to such person or persons
          so appointed in writing by you, or otherwise to your legal
          representatives or estate.

                                       11
<PAGE>

18.  ENTIRE AGREEMENT
     ----------------

     Other than the Proprietary Agreement, your stock option agreements and
     applicable stock option plans, and the CIC Plan, this Agreement, the
     Resignation Letter attached hereto as Appendix A, and the General Release
     attached hereto as Appendix B constitute the entire agreement, arrangement
     and understanding between you and Amgen; they may not be modified or
     canceled in any manner except by a writing signed by both you and Amgen.
     This Agreement and the General Release supersede any prior or
     contemporaneous agreement, arrangement or understanding on this subject
     matter.  By executing this Agreement, the Resignation Letter, and the
     General Release below, you expressly acknowledge the termination of any
     such prior agreement, arrangement or understanding.  Also, by executing
     this Agreement, the Resignation Letter, and the General Release, you affirm
     that no one has made any written or verbal statement that contradicts the
     provisions of this Agreement, the Resignation Letter, or the General
     Release.


                                    Sincerely yours,

                                    /s/ Gordon M. Binder
                                    ------------------------------------
                                    Amgen Inc.
                                    By: Gordon M. Binder
                                    Chief Executive Officer and Chairman

Acknowledged and Agreed:

/s/ Dr. N. Kirby Alton
-----------------------
    Dr. N. Kirby Alton                                 Dated: 10/11/99
                                                             ---------

                                       12
<PAGE>

                                  APPENDIX A


            [To be printed on Dr. N. Kirby Alton's Amgen Stationery]



                                  RESIGNATION
                                  -----------


The undersigned hereby resigns as an officer and/or director of the following
corporations, effective October 20, 1999:

     Amgen Inc.
     Kirin Amgen, Inc.


                               /s/ Dr. N. Kirby Alton
                               ---------------------------------
                                   Dr. N. Kirby Alton
<PAGE>

                                   APPENDIX B

                             MUTUAL GENERAL RELEASE

     By signing below, Amgen Inc. ("Amgen" or the "Company") and you, Dr. N.
Kirby Alton, agree to all of the terms and conditions set forth in this Mutual
General Release, which resolves all issues between you and the Company
including, but not limited to, those related to your employment with the
Company, and the termination thereof.

1.   COMPLETE RELEASE
     ----------------

     1.1  Release:  In exchange for consideration provided to you and the
          -------
          Company under the Agreement, the receipt of which and adequacy thereof
          you and the Company hereby acknowledge, you irrevocably and
          unconditionally release all the claims described in Subparagraph 1.2
          of this General Release that you may have against the following
          persons or entities (collectively the "Amgen Releasees"): Amgen, all
          related or affiliated companies and all of Amgen's or such related or
          affiliated companies' predecessors, successors, and assigns; and, with
          respect to each such entity, all of its past and present employees,
          officers, directors, stockholders, owners, representatives, assigns,
          attorneys, agents, insurers, employee benefit programs (and the
          trustees, administrators, fiduciaries and insurers of such programs)
          and any other persons acting by, through, under or in concert with any
          of the persons or entities listed in this Subparagraph and each of
          them; and the Company irrevocably and unconditionally releases all the
          claims described in Subparagraph 1.2 of this General Release that the
          Company may have against you, your employees, agents, attorneys,
          representatives, successors, and assigns, past and present and each of
          them.

     1.2  Claims Released:  Except as provided in Subparagraph 1.4 of this
          ---------------
          General Release, the claims released include all claims of whatever
          nature, whether known or unknown, suspected or unsuspected, by either
          you or Amgen which you or Amgen now owns or holds or has at any time
          previously held, or (with the sole exception of claims covered by
          Subparagraph 1.4 of this General Release) ever in the future may hold
          including statutory claims arising under the employment discrimination
          laws.  In particular, you acknowledge and agree that by signing the
          Agreement and this General Release, in addition to the matters
          discussed above, you are waiving and releasing any and all claims,
          charges, or rights you may have under the Age Discrimination In
          Employment Act of 1967, as amended (the "ADEA"), that this waiver and
          release is knowing and voluntary, and that the consideration given for
          this waiver and release is in addition to anything of value to which
          you were already entitled as an employee of Amgen.  You further
          acknowledge that you have been advised that:  (a) you should consult
          with an attorney (at your own expense, subject to your right to
          reimbursement as set forth in Paragraph 5 of the Agreement) prior to
          executing the Agreement and this General Release; (b) you have at
          least twenty-one (21) days in which to consider the Agreement and this
          General Release (although you may choose to execute the Agreement and
          this General Release earlier and waive all of or part of the 21-day
          period); (c) the Agreement and this General
<PAGE>

          Release do not waive or release any rights or claims you may have
          under the ADEA which may arise after you execute the Agreement and
          this General Release; (d) you have seven (7) days following execution
          of the Agreement and this General Release to revoke your consent to
          the Agreement and this General Release (to be effective, any
          revocation must be actually received in writing by me by 5:30 p.m. on
          the seventh day); and (e) the Agreement and this General Release shall
          not be effective until the seven (7) day revocation period has
          expired. In the event that you exercise this right to revoke this
          General Release, you and Amgen agree that the Agreement (including
          without limitation the Resignation Letter attached to the Agreement as
          Appendix A) will be simultaneously revoked. You also acknowledge and
          agree that you were given a copy of the Agreement and this General
          Release on September 21, 1999, that you have been given the
          opportunity to consult with whomever you wish regarding the Agreement
          and this General Release and that you have entered into the Agreement
          and this General Release voluntarily and with full knowledge of its
          final and binding effect.

     1.3  Release Extends to Both Known and Unknown Claims:  This General
          ------------------------------------------------
          Release covers both claims that you and/or Amgen know about and those
          you and/or Amgen do not know about.  You understand the significance
          of this release of unknown claims and this waiver of statutory
          protection against a release of unknown claims by both you and Amgen.
          You and Amgen each expressly waive all rights afforded by any statute
          which limits the effect of a release with respect to unknown claims.
          You and Amgen each expressly waive the protection of (S) 1542 of the
          Civil Code of the State of California.

     1.4  Claims Not Released:  This General Release does not release your right
          -------------------
          or the Company's right to enforce the Agreement.

2.   YOUR PROMISES
     -------------

     In addition to the release of claims provided for in Paragraph 1 of this
     General Release, you also agree to the following:

     2.1  No Future Employment:  You understand that, as provided in Paragraph 8
          --------------------
          of the Agreement, your employment with Amgen and all related or
          affiliated companies will terminate forever on the Termination Date
          and you promise never to seek employment with Amgen or its related or
          affiliated companies in the future.  If your employment is not
          terminated by Amgen for a Stated Reason, Amgen shall treat this
          termination as a resignation on its records.  You acknowledge and
          agree that the Agreement, together with this General Release,
          contemplates your termination from Amgen on the Termination Date, and
          that the release in Paragraph 1 of this General Release shall cover
          your entire employment with Amgen and the termination of that
          employment.  On the day after the Termination Date, you and Amgen each
          promise to execute and be bound by a second General Release which
          contains all of the provisions set forth in this General Release and
          which reconfirms each party's release of such claims as of the
          Termination Date.

                                      B-2
<PAGE>

     2.2  You are Not to Harm Amgen:  You agree not to knowingly and willfully
          -------------------------
          criticize, denigrate or otherwise disparage Amgen, any other Amgen
          Releasee, or any of Amgen's products, processes, experiments,
          policies, practices, standards of business conduct, or areas or
          techniques of research to the extent that such conduct causes
          demonstrable injury to Amgen; provided, however, that nothing in this
          General Release shall prohibit you from complying with any lawful
          subpoena or court order.

     2.3  No Knowledge of Violations:  You represent that you are not aware of
          --------------------------
          any facts that would (a) establish, (b) tend to establish, or (c) in
          any way support an allegation of a violation by Amgen of the federal
          False Claims Act (or any similar state or federal qui tam statute).
                                                            --- ---

3.  CONSEQUENCES OF YOUR VIOLATION OF YOUR PROMISES
    -----------------------------------------------

     3.1  General Consequences:  If you break any of the promises made in the
          --------------------
          Agreement or this General Release, for example, by filing or
          prosecuting a lawsuit based on claims that you have released, or
          declining to execute the second General Release contemplated by
          Paragraph 2.1, or if any representation made by you in this General
          Release was false when made, you (a) shall forfeit all right to future
          benefits under the Agreement; (b) must repay all benefits previously
          received, other than the monthly compensation paid to you under
          Paragraph 2.1 of the Agreement, upon Amgen's demand; and (c) must pay
          reasonable attorneys' fees and all other costs incurred as a result of
          your breach or false representation, such as the cost of defending any
          suit brought with respect to a released claim by you or other owner of
          a released claim.  It is agreed that your breach of Subparagraph 2.2
          of this Agreement will not be covered by this Paragraph 3.1 unless you
          are given written notice by the Company specifying your breach of
          Subparagraph 2.2 and you fail to cure such a breach within 14 days of
          receipt of such notice.

          In addition, in order to ensure that you have complied fully with your
          obligations under Paragraph 2.3 of this General Release, you hereby
          covenant and agree that to the full extent permitted by law, you
          hereby waive and release any and all rights or claims you may have to
          any personal claim for proceeds or awards that you may be entitled to
          under any qui tam proceeding brought against Amgen.  You further agree
                    --- ---
          that you shall deliver any such money, proceeds, or awards to the U.S.
          government.

     3.2  Injunctive Relief:  You further agree that Amgen would be irreparably
          -----------------
          harmed by any use or disclosure of information that is prohibited by
          the Amgen Inc. Proprietary Information and Inventions Agreement,
          executed by you on or about January 15, 1982 (the "Proprietary
          Agreement") (which contains obligations that survive the termination
          of your employment with Amgen), and that Amgen shall be entitled to an
          injunction prohibiting you from committing any such violation.

     3.3  Challenges to Validity:  Should you attempt to challenge the formation
          ----------------------
          or enforceability of the Agreement and/or this General Release, you
          shall initially

                                      B-3
<PAGE>

          tender, by certified check delivered to Amgen, all amounts received
          pursuant to the Agreement, other than the monthly compensation paid to
          you under Paragraph 2.1 of the Agreement, plus interest at the legal
          rate and invite Amgen to cancel the Agreement. In the event Amgen
          accepts this offer, the Agreement shall be canceled. In the event
          Amgen does not accept this offer, Amgen shall so notify you and the
          amount tendered by you shall be placed in an interest-bearing account
          pending a determination of the enforceability of the Agreement and/or
          this General Release. If the Agreement and this General Release are
          determined to be enforceable, the amount in the account shall be
          repaid to you; if the Agreement and/or this General Release are
          determined not to be enforceable, the amount in the account shall be
          retained by Amgen or its designee.

4.   VOLUNTARILY ENTERING AGREEMENT
     ------------------------------

     You acknowledge that you (a) have had a sufficient period to consider and
     review the Agreement and this General Release before signing them; (b) have
     carefully read the Agreement and this General Release; and (c) fully
     understand the Agreement and this General Release and are entering into
     them voluntarily.

5.   SEVERABILITY
     ------------

     The provisions of this General Release are severable. If any one or more of
     the provisions contained herein, or the application thereof in any
     circumstance, is held invalid, illegal or unenforceable in any respect and
     for any reason, the validity, legality and enforceability of any such
     provision in every other respect and of the remaining provisions hereof
     shall not be affected or impaired in any way, it being intended that all of
     the parties' rights and privileges arising hereunder shall be enforceable
     to the fullest extent permitted by law.

PLEASE READ THIS GENERAL RELEASE CAREFULLY.  IT CONTAINS A RELEASE OF ALL KNOWN
AND UNKNOWN CLAIMS.

Executed at Thousand Oaks, California this 11th day of October, 1999.
                                           ----        -------


/s/ Dr. N. Kirby Alton
---------------------
Dr. N. Kirby Alton


Executed at Thousand Oaks, California this 11th day of October, 1999.
                                           ----        -------



/s/ Gordon M. Binder
--------------------
Amgen Inc.
By:  Gordon M. Binder
Chief Executive Officer and Chairman

                                      B-4