<PAGE>

                                                                   EXHIBIT 10.99

                                                                         Annex B

                           PERFORMANCE UNIT AGREEMENT

__________________, Amgen Inc. Grantee:

                  On this __ day of _______________ (the "Grant Date"), Amgen
Inc., a Delaware corporation (the "Company"), pursuant to its Performance Award
Program (the "Program") which implements the Amended and Restated 1991 Equity
Incentive Plan (the "Plan"), has granted to you, the grantee named above,
_____________ performance units (the "Units") on the terms and conditions set
forth in this Performance Unit Agreement (this "Agreement"), the Plan, the
Program and the Resolutions (as defined below). Capitalized terms not defined
herein shall have the meanings assigned to such terms in the Program.

                  I.       Performance Cycle. The Performance Cycle shall begin
on January 1, 200__ and end on December 31, 200__.

                  II.      Value of Units. The value of each Unit is equal to
the closing price of a share of Common Stock on the Grant Date.

                  III.     Performance Goals. Up to 225% of the Units shall be
earned, depending on the extent to which the Company achieves objectively
determinable performance goals established by the Compensation and Management
Development Committee (the "Committee") pursuant to those certain Resolutions of
the Compensation and Management Development Committee of the Board of Directors
of Amgen Inc., adopted on March 8, 2004, regarding the Performance Award Program
(the "Resolutions"). The Units earned shall be calculated in accordance with the
Resolutions and the Program.

                  IV.      Form and Timing of Payment. Subject to Section X and
except as set forth in the Program, any Units earned pursuant to Section III
above shall be paid as soon as practicable following the Determination Date in
shares of Common Stock based on a 30-day average trading price of the Common
Stock ending seven trading days immediately preceding the Determination Date;
provided, however, that no shares of Common Stock shall be issued hereunder
unless the Board determines prior to such issuance that the consideration
received by the Company in exchange for the issuance of Common Stock has a value
not less than the par value thereof.

                  V.       Issuance of Certificates; Tax Withholding. You shall
satisfy any federal, state and local tax withholding obligation relating to the
payment of the Units earned by hereby authorizing the Company to withhold from
the shares of the Common Stock otherwise issuable to you as a result of the
vesting or the payment of the Units earned a number of shares having a fair
market value less than or equal to the amount of the Company's required minimum
statutory withholding. Any shares of Common Stock withheld by the Company
hereunder shall not be deemed to have been issued by the Company for any purpose
under the Plan. In addition, you

<PAGE>

shall take any further actions and execute any additional documents as may be
necessary to effectuate the provisions of this Section V. Notwithstanding
Section IV above, no certificates representing the shares of Common Stock shall
be delivered to you unless and until you have paid to the Company the full
amount of all federal, state and local tax withholding or other employment taxes
applicable to you resulting from the payment of the Units earned.

                  VI.      Nontransferability. No benefit payable under, or
interest in, this Agreement shall be subject in any manner to anticipation,
alienation, sale, transfer, assignment, pledge, encumbrance or charge and any
such attempted action shall be void and no such benefit or interest shall be, in
any manner, liable for, or subject to, your or your beneficiary's debts,
contracts, liabilities or torts; provided, however, nothing in this Section VI
shall prevent transfer (i) by will, (ii) by applicable laws of descent and
distribution or (iii) to an Alternate Payee to the extent that a QDRO so
provides, as further described in the Program.

                  VII.     No Contract for Employment. This Agreement is not an
employment or service contract and nothing in this Agreement shall be deemed to
create in any way whatsoever any obligation on your part to continue in the
employ or service of the Company, or of the Company to continue your employment
or service with the Company.

                  VIII.    Notices. Any notices provided for in this Agreement
or the Plan shall be given in writing and shall be deemed effectively given upon
receipt or, in the case of notices delivered by the Company to you, five (5)
days after deposit in the United States mail, postage prepaid, addressed to you
at such address as is currently maintained in the Company's records or at such
other address as you hereafter designate by written notice to the Company.

                  IX.      Resolutions, Plan and Program. This Agreement is
subject to all the provisions of the Resolutions, the Plan and the Program and
their provisions are hereby made a part of this Agreement, including without
limitation the provisions of Sections 7 and 10(d) of the Plan (relating to stock
bonuses) and Section 11 of the Plan (relating to adjustments upon changes in the
Common Stock), and is further subject to all interpretations, amendments, rules
and regulations which may from time to time be promulgated and adopted pursuant
to the Plan. In the event of any conflict between the provisions of this
Agreement and those of the Resolutions, the Plan and the Program, the provisions
of the Plan shall control. Notwithstanding any provision of this Agreement or
the Program to the contrary, any earned Units paid in cash rather than shares of
Common Stock shall not be deemed to have been issued by the Company for any
purpose under the Plan.

                  X.       Provisions Applicable to Participants in Foreign
Jurisdictions. Notwithstanding any provision of this Agreement or the Program to
the contrary, if you are employed by the Company or its Affiliates outside the
United States or are subject to the laws of any foreign jurisdiction, your award
of Units shall be subject to the following additional terms and conditions:

                  (a)      the terms and conditions of your award of Units are
deemed modified to the extent necessary to comply with applicable foreign laws;

<PAGE>

                  (b)      if applicable, the effectiveness of your award of
Units is conditioned upon approval or compliance with any necessary local
governmental regulatory exemption or approvals;

                  (c)      to the extent necessary to comply with applicable
foreign laws, the payment of any earned Units shall be made in cash or Common
Stock, at the Company's election; and

                  (d)      the Committee may take any other action, before or
after an award of Units is made, that it deems advisable to obtain approval or
comply with any necessary local governmental regulatory exemptions or approvals.

Notwithstanding the foregoing, the Committee may not take any actions hereunder,
and no award of Units shall be granted, that would violate the Exchange Act, the
Code, any securities law or governing statute or any other applicable law.

                  XI.      Governing Law. This Agreement shall be construed and
interpreted, and the rights of the parties shall be determined, in accordance
with the laws of the State of Delaware, without regard to conflicts of law
provisions thereof.

                                                   Very truly yours,
                                                   AMGEN INC.

                                                   By:__________________________
                                                   Name:
                                                   Title:

Accepted and Agreed,
this ___ day of ___________, 2004.

By:________________________
Name: