Document:

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                                                                   Exhibit 10.20

                                    ADDENDUM
                                     TO THE
                               IMMUNEX CORPORATION
                             1999 STOCK OPTION PLAN
                   (As Amended and Restated on April 25, 2000)

               WHEREAS, Immunex Corporation (hereinafter referred to as the
"Company") maintains the Immunex Corporation 1999 Stock Option Plan (hereinafter
referred to as the "Plan") and the Stock Option Grant Program for Nonemployee
Directors under the Plan (the "Program"); and

               WHEREAS, the Company desires to adjoin an addendum (this
"Addendum") to the Plan to address the effects of the transactions contemplated
by the Agreement and Plan of Merger by and between Amgen Inc., AMS Acquisition
Inc. and the Company dated as of December 16, 2001 (the "Merger Agreement");

               NOW, THEREFORE, notwithstanding anything in the Plan and the
Program to the contrary, this Addendum is hereby adopted, effective as of the
Effective Time (as defined in the Merger Agreement):

        Section 1. At the Effective Time, each option granted pursuant the Plan
shall be treated in accordance with the applicable terms of the Merger
Agreement.

        Section 2. In the event that an optionee's employment with the Company
or Amgen Inc. is terminated by the optionee for Good Reason or by the Company or
Amgen Inc. without Cause during the fifteen (15) months following the Effective
Time, each option held by such optionee for common stock of Amgen Inc. that
granted pursuant to the Merger Agreement with respect to (a) a Cancelled Company
Option (as defined in the Merger Agreement) or (b) an option for common stock of
the Company that was granted after December 16, 2001, shall immediately vest in
full and shall remain exercisable until the earlier of (x) the first anniversary
of the optionee's termination of employment or (y) the end of the term of such
option.

        Section 3. In the event that an optionee who is a nonemployee director
of the Company immediately prior to the Effective Time ceases to be a director
of the Company or Amgen Inc. for any reason immediately prior to, at, or during
the fifteen (15) months following the Effective Time, each option held by such
optionee for common stock of Amgen Inc. shall immediately vest in full and shall
remain exercisable until the earlier of (x) the first anniversary of the date
such optionee ceases to be a director of the Company or Amgen Inc. or (y) the
end of the term of such option.

        Section 4. For purposes of this Addendum only, "Good Reason" shall mean
the occurrence on or after the Effective Date and without the optionee's consent
of, (a) a reduction in the optionee's annual base salary or wages, other than as
part of a general reduction applicable to substantially all employees of the
Company employed in the United States or (ii) the relocation of the optionee's
principal place of employment to a location more than fifty (50) miles from the
optionee's principal place of employment prior to the Effective Date.

        Section 5. For purposes of this Addendum only, "Cause" shall mean (a)
the willful and continued failure by the optionee to substantially perform the
optionee's duties with the Company (other than such failure resulting form the
optionee's incapacity due to physical or mental illness) or (b) the willful
engaging by the optionee in conduct which is demonstratably and materially
injurious to the Company, monetarily or otherwise. For purposes of this
definition, no act, or failure to act, on the optionee's part shall be deemed
willful unless done, or omitted to be done, by the optionee not in good faith or
without reasonable belief that the optionee's act, or failure to act, was in the
best interest of the Company.

        Section 6. This Addendum shall be effective only upon the Effective
Time. In the event that the Merger Agreement terminates according to its terms,
this Addendum shall be of no force or effect.<PAGE>

                                                                   Exhibit 10.30

                               SEVERANCE AGREEMENT
                               -------------------

                  THIS AGREEMENT, dated as of December 16, 2001, is made by and
between Immunex Corporation, a Washington corporation (the "Company"), and Peggy
V. Phillips (the "Executive").

                  WHEREAS, the Company considers it essential to the best
interests of its shareholders to foster the continued employment of key
management personnel; and

                  WHEREAS, the Board recognizes that, as is the case with many
publicly held corporations, the possibility of a Change in Control exists and
that such possibility, and the uncertainty and questions which it may raise
among management, may result in the departure or distraction of management
personnel to the detriment of the Company and its shareholders; and

                  WHEREAS, the Board has determined that appropriate steps
should be taken to reinforce and encourage the continued attention and
dedication of members of the Company's management, including the Executive, to
their assigned duties without distraction in the face of potentially disturbing
circumstances arising from the possibility of a Change in Control;

                  NOW, THEREFORE, in consideration of the premises and the
mutual covenants herein contained, the Company and the Executive hereby agree as
follows:

                  1.   Defined Terms.  The definitions of capitalized terms used
                       -------------
in this Agreement are provided in the last Section hereof.

                  2.   Term of Agreement. The Term of this Agreement shall
                       -----------------
commence on the date hereof and shall continue in effect through December 31,
2004; provided, however, that commencing on January 1, 2002 and each January 1
      --------  -------
thereafter, the Term shall automatically be extended for one additional year
unless, not later than September 30 of the preceding year, the Company or the
Executive shall have given notice not to extend the Term; and further provided,
                                                              ------- --------
however, that if a Change in Control shall have occurred during the Term, the
-------
Term shall expire on the date which is twenty-four (24) months immediately
following the date on which such Change in Control occurred.

                  3.   Company's Covenants Summarized. In order to induce the
                       ------------------------------
Executive to remain in the employ of the Company and in consideration of the
Executive's covenants set forth in Section 4 hereof, the Company agrees, under
the conditions described herein, to pay the Executive the Severance Payments and
the other payments and benefits described herein. Except as provided in Section
9.1 hereof, no Severance Payments shall be payable under this Agreement unless
there shall have been a termination of the Executive's employment with the
Company under the circumstances described herein within the two year period
immediately following a Change in Control that occurs during the Term. This
Agreement shall not be construed as creating an express or implied contract of
employment and, except as otherwise agreed in writing between the Executive and
the Company, the Executive shall not have any right to be retained in the employ
of the Company.

                  4.   The Executive's Covenants. The Executive agrees that,
                       -------------------------
subject to the terms and conditions of this Agreement, in the event of a
Potential Change in Control during the Term, the Executive will

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remain in the employ of the Company until the earliest of (i) a date which is
six (6) months from the date of such Potential Change in Control, (ii) the date
of a Change in Control, (iii) the date of termination by the Executive of the
Executive's employment for Good Reason or by reason of death, Disability or
Retirement, or (iv) the termination by the Company of the Executive's employment
for any reason.

                  5.   Compensation Other Than Severance Payments.
                       ------------------------------------------

                  5.1  If the Executive's employment shall be terminated for any
reason following a Change in Control and during the Term, the Company shall pay
the Executive's full salary to the Executive through the Date of Termination at
the rate in effect immediately prior to the Date of Termination or, if higher,
the rate in effect immediately prior to the Change in Control, together with all
compensation and benefits payable to the Executive through the Date of
Termination under the terms of the Company's compensation and benefit plans,
programs or arrangements as in effect immediately prior to the Date of
Termination or, if more favorable to the Executive, as in effect immediately
prior to the Change in Control.

                  5.2  If the Executive's employment shall be terminated for any
reason following a Change in Control and during the Term, the Company shall pay
to the Executive the Executive's normal post-termination compensation and
benefits as such payments become due. Such post-termination compensation and
benefits shall be determined under, and paid in accordance with, the Company's
retirement, insurance and other compensation or benefit plans, programs and
arrangements as in effect immediately prior to the Date of Termination or, if
more favorable to the Executive, as in effect immediately prior to the Change in
Control.

                  6.   Severance Payments.
                       ------------------

                  6.1  If the Executive's employment is terminated within the
two year period immediately following a Change in Control that occurs during the
Term, other than (A) by the Company for Cause, (B) by reason of death or
Disability, or (C) by the Executive without Good Reason, then, subject to
Section 6.5, the Company shall pay the Executive the amounts, and provide the
Executive the benefits, described in this Section 6.1 ("Severance Payments") and
Section 6.2, in addition to any payments and benefits to which the Executive is
entitled under Section 5 hereof.

                       (A)    In lieu of any further salary payments to the
         Executive for periods subsequent to the Date of Termination and in lieu
         of any severance benefit otherwise payable to the Executive including
         amounts payable pursuant to the Immunex Corporation Leadership
         Continuity Plan (but not in lieu of amounts payable pursuant to the
         Immunex Corporation Retention Plan, except as expressly provided in
         such plan), the Company shall pay to the Executive a lump sum severance
         payment, in cash, equal to three times the sum of (i) the Executive's
         base salary as in effect immediately prior to the Date of Termination
         or, if higher, in effect immediately prior to the first occurrence of
         an event or circumstance constituting Good Reason, (ii) the target
         annual incentive compensation in effect immediately prior to the Date
         of Termination or, if higher, in effect immediately prior to the first
         occurrence of event or circumstance constituting Good Reason, and (iii)
         the value of the contributions or the allocations made, as applicable,
         on behalf of the Executive to any DC Pension Plan in respect of the
         fiscal year ending immediately prior to the fiscal year in which occurs
         the Date of Termination or, if higher, immediately prior to the fiscal
         year in which occurs the first event or circumstance constituting Good
         Reason.

                       (B)    For the thirty-six (36) month period immediately
         following the Date of Termination, the Company shall arrange to provide
         the Executive and his dependents (a) life, death,

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         accident and health insurance benefits substantially similar to those
         provided to the Executive and his dependents immediately prior to the
         Date of Termination or, if more favorable to the Executive, those
         provided to the Executive and his dependents immediately prior to the
         first occurrence of an event or circumstance constituting Good Reason,
         at no greater cost after tax to the Executive than the after tax cost
         to the Executive immediately prior to such date or occurrence; and (b)
         other perquisites, including, without limitation, financial counseling
         and tax planning services by AYCO or a company providing comparable
         equivalent services, to the same extent as if the Executive had
         continued to be employed by the Company during such period. Benefits
         otherwise receivable by the Executive pursuant to this Section
         6.1(B)(a) shall be reduced to the extent benefits of the same type are
         received by or made available to the Executive during the thirty-six
         (36) month period following the Executive's termination of employment
         (and any such benefits received by or made available to the Executive
         shall be reported to the Company by the Executive); provided, however,
                                                             --------  -------
         that the Company shall reimburse the Executive for the excess, if any,
         of the cost of such benefits to the Executive over such cost
         immediately prior to the Date of Termination or, if more favorable to
         the Executive, the first occurrence of an event or circumstance
         constituting Good Reason.

                       (C)    The Company shall provide the Executive with
         outplacement services suitable to the Executive's position for a period
         of one year commencing on the date the Executive first uses such
         outplacement services.

                  6.2  (A)    Whether or not the Executive becomes entitled to
the Severance Payments, if any of the payments or benefits received or to be
received by the Executive in connection with a Change in Control or the
Executive's termination of employment (whether pursuant to the terms of this
Agreement or any other plan, arrangement or agreement with the Company, any
Person whose actions result in a Change in Control or any Person affiliated with
the Company or such Person) (all such payments and benefits, excluding the
Gross-Up Payment, being hereinafter referred to as the "Total Payments") will be
subject to the Excise Tax, the Company shall pay to the Executive an additional
amount (the "Gross-Up Payment") such that the net amount retained by the
Executive, after deduction of any Excise Tax on the Total Payments and any
federal, state and local income and employment taxes and Excise Tax upon the
Gross-Up Payment, and after taking into account the phase out of itemized
deductions and personal exemptions attributable to the Gross-Up Payment, shall
be equal to the Total Payments.

                  (B)  For purposes of determining whether any of the Total
Payments will be subject to the Excise Tax and the amount of such Excise Tax,
(i) all of the Total Payments shall be treated as "parachute payments" (within
the meaning of section 280G(b)(2) of the Code) unless, in the opinion of tax
counsel ("Tax Counsel") reasonably acceptable to the Executive and selected by
the accounting firm which was, immediately prior to the Change in Control, the
Company's independent auditor (the "Auditor"), such payments or benefits (in
whole or in part) do not constitute parachute payments, including by reason of
section 280G(b)(4)(A) of the Code, (ii) all "excess parachute payments" within
the meaning of section 280G(b)(l) of the Code shall be treated as subject to the
Excise Tax unless, in the opinion of Tax Counsel, such excess parachute payments
(in whole or in part) represent reasonable compensation for services actually
rendered (within the meaning of section 280G(b)(4)(B) of the Code) in excess of
the Base Amount allocable to such reasonable compensation, or are otherwise not
subject to the Excise Tax, and (iii) the value of any noncash benefits or any
deferred payment or benefit shall be determined by the Auditor in accordance
with the principles of sections 280G(d)(3) and (4) of the Code. For purposes of
determining the amount of the Gross-Up Payment, the Executive shall be deemed to
pay federal income tax at the highest marginal rate of federal income taxation
in the calendar year in which the Gross-Up Payment is to be made and state and
local income taxes at the highest marginal rate of taxation in the state and
locality of the Executive's residence on the Date of Termination (or if there is
no Date of Termination, then the date on which the Gross-Up Payment is
calculated for purposes of this Section 6.2), net of the maximum reduction in
federal income taxes which could be obtained from deduction of such state and
local taxes.

<PAGE>

                  (C)  In the event that the Excise Tax is finally determined
to be less than the amount taken into account hereunder in calculating the
Gross-Up Payment, the Executive shall repay to the Company, within five (5)
business days following the time that the amount of such reduction in the Excise
Tax is finally determined, the portion of the Gross-Up Payment attributable to
such reduction (plus that portion of the Gross-Up Payment attributable to the
Excise Tax and federal, state and local income and employment taxes imposed on
the Gross-Up Payment being repaid by the Executive), to the extent that such
repayment results in a reduction in the Excise Tax and a dollar-for-dollar
reduction in the Executive's taxable income and wages for purposes of federal,
state and local income and employment taxes, plus interest on the amount of such
repayment at 120% of the rate provided in section 1274(b)(2)(B) of the Code. In
the event that the Excise Tax is determined to exceed the amount taken into
account hereunder in calculating the Gross-Up Payment (including by reason of
any payment the existence or amount of which cannot be determined at the time of
the Gross-Up Payment), the Company shall make an additional Gross-Up Payment in
respect of such excess (plus any interest, penalties or additions payable by the
Executive with respect to such excess) within five (5) business days following
the time that the amount of such excess is finally determined. The Executive and
the Company shall each reasonably cooperate with the other in connection with
any administrative or judicial proceedings concerning the existence or amount of
liability for Excise Tax with respect to the Total Payments.

                  6.3  The payments provided in subsection (A) of Section 6.1
hereof and in Section 6.2 hereof shall be made not later than the fifth day
following the Date of Termination (or if there is no Date of Termination, then
the date on which the Gross-Up Payment is calculated for purposes of Section 6.2
hereof); provided, however, that if the amounts of such payments cannot be
         --------  -------
finally determined on or before such day, the Company shall pay to the Executive
on such day an estimate, as determined in good faith by the Company or, in the
case of payments under Section 6.2 hereof, in accordance with Section 6.2
hereof, of the minimum amount of such payments to which the Executive is clearly
entitled and shall pay the remainder of such payments (together with interest on
the unpaid remainder (or on all such payments to the extent the Company fails to
make such payments when due) at 120% of the rate provided in section
1274(b)(2)(B) of the Code) as soon as the amount thereof can be determined but
in no event later than the thirtieth (30th) day after the Date of Termination.
In the event that the amount of the estimated payments exceeds the amount
subsequently determined to have been due, such excess shall constitute a loan by
the Company to the Executive, payable on the fifth (5th) business day after
demand by the Company (together with interest at 120% of the rate provided in
section 1274(b)(2)(B) of the Code). Within fifteen (15) days prior to the time
that payments are made under this Agreement, Tax Counsel shall provide the
Company and the Executive with a written statement setting forth the manner in
which such payments were calculated and the basis for such calculations.

                  6.4  The Company also shall pay to the Executive all legal
fees and expenses incurred by the Executive (i) in disputing in good faith any
issue hereunder relating to the termination of the Executive's employment, in
seeking in good faith to obtain or enforce any benefit or right provided by this
Agreement, provided that the Executive prevails in any material dispute, or (ii)
in connection with any tax audit or proceeding to the extent attributable to the
application of section 4999 of the Code to any payment or benefit provided
hereunder. Such payments shall be made within five (5) business days after
delivery of the Executive's written requests for payment accompanied with such
evidence of fees and expenses incurred as the Company reasonably may require.

                  6.5  Notwithstanding anything contained herein, the Executive
shall not be entitled to receive the Severance Payments or any other payment or
benefit hereunder unless he or she first executes a written release in the form
attached hereto as Exhibit A and such release has become effective.

                  7.   Termination Procedures and Compensation During Dispute.
                       ------------------------------------------------------

<PAGE>

                  7.1  Notice of Termination. After a Change in Control and
                       ---------------------
during the Term, any purported termination of the Executive's employment (other
than by reason of death) shall be communicated by written Notice of Termination
from one party hereto to the other party hereto in accordance with Section 10
hereof. For purposes of this Agreement, a "Notice of Termination" shall mean a
notice which shall indicate the specific termination provision in this Agreement
relied upon and shall set forth in reasonable detail the facts and circumstances
claimed to provide a basis for termination of the Executive's employment under
the provision so indicated. Further, a Notice of Termination for Cause is
required to include a copy of a resolution duly adopted by the affirmative vote
of not less than three quarters (3/4) of the entire membership of the Committee
at a meeting of the Committee which was called and held for the purpose of
considering such termination (after reasonable notice to the Executive and an
opportunity for the Executive, together with the Executive's counsel, to be
heard before the Committee) finding that, in the good faith opinion of the
Committee, the Executive was guilty of conduct set forth in clause (i) or (ii)
of the definition of Cause herein, and specifying the particulars thereof in
detail.

                  7.2  Date of Termination. "Date of Termination," with respect
                       -------------------
to any purported termination of the Executive's employment after a Change in
Control and during the Term, shall mean (i) if the Executive's employment is
terminated for Disability, fifteen (15) days after Notice of Termination is
given (provided that the Executive shall not have returned to the full-time
performance of the Executive's duties during such fifteen (15) day period), and
(ii) if the Executive's employment is terminated for any other reason, the date
specified in the Notice of Termination (which, in the case of a termination by
the Company, shall not be less than fifteen (15) days (except in the case of a
termination for Cause) and, in the case of a termination by the Executive, shall
not be less than fifteen (15) days nor more than thirty (30) days, respectively,
from the date such Notice of Termination is given).

                  8.   No Mitigation. The Company agrees that, if the
                       -------------
Executive's employment with the Company terminates during the Term, the
Executive is not required to seek other employment or to attempt in any way to
reduce any amounts payable to the Executive by the Company pursuant to Section 6
hereof. Further, except as specifically provided in this Section 8 and Section
6.1(B) hereof, the amount of any payment or benefit provided for in this
Agreement shall not be reduced by any compensation earned by the Executive as
the result of employment by another employer, by retirement benefits, by offset
against any amount claimed to be owed by the Executive to the Company, or
otherwise. Notwithstanding anything herein to the contrary, if the Executive's
employment is terminated by the Executive with Good Reason during the sixty (60)
day period following the first anniversary of a Change in Control as described
in the last sentence of the first paragraph of Section 15(Q) hereof for reasons
which would not otherwise constitute Good Reason, payments provided in
subsection (A) of Section 6.1 hereof and in Section 6.2 hereof shall be reduced
by any Retention Payment previously paid under the Immunex Corporation Retention
Plan.

                  9.   Successors; Binding Agreement.
                       -----------------------------

                  9.1  In addition to any obligations imposed by law upon any
successor to the Company, the Company 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 the Company to expressly
assume and agree to perform this Agreement in the same manner and to the same
extent that the Company would be required to perform if no such succession had
taken place. Failure of the Company to obtain such assumption and agreement upon
or prior to the effectiveness of any such succession shall be a breach of this
Agreement and shall entitle the Executive to terminate his employment with the
Company and receive compensation from the Company in the same amount and on the
same terms as the Executive would be entitled to hereunder if the Executive were
to terminate the Executive's employment for Good Reason after a Change in
Control, except

<PAGE>

that, for purposes of implementing the foregoing, the date on which any such
succession becomes effective shall be deemed the Date of Termination.

                  9.2  This Agreement shall inure to the benefit of and be
enforceable by the Executive's personal or legal representatives, executors,
administrators, successors, heirs, distributees, devisees and legatees. If the
Executive shall die while any amount would still be payable to the Executive
hereunder (other than amounts which, by their terms, terminate upon the death of
the Executive) if the Executive had continued to live, all such amounts, unless
otherwise provided herein, shall be paid in accordance with the terms of this
Agreement to the executors, personal representatives or administrators of the
Executive's estate.

                  10.  Notices. For the purpose of this Agreement, notices and
                       -------
all other communications provided for in the Agreement shall be in writing and
shall be deemed to have been duly given when delivered or mailed by United
States registered mail, return receipt requested, postage prepaid, addressed, if
to the Executive, to the address inserted below the Executive's signature on the
final page hereof and, if to the Company, to the address set forth below, or to
such other address as either party may have furnished to the other in writing in
accordance herewith, except that notice of change of address shall be effective
only upon actual receipt:

                       To the Company:

                               Immunex Corporation

                              51 University Street

                         Seattle, Washington 98101-2936

                           Attention: General Counsel

                  11.  Miscellaneous. No provision of this Agreement may be
                       -------------
modified, waived or discharged unless such waiver, modification or discharge is
agreed to in writing and signed by the Executive and any authorized officer of
the Company. No waiver by either party hereto at any time of any breach by the
other party hereto of, or of any lack of compliance with, any condition or
provision of this Agreement to be performed by such other party shall be deemed
a waiver of similar or dissimilar provisions or conditions at the same or at any
prior or subsequent time. Except for the Immunex Corporation Retention Plan
(which remains in full force and effect), this Agreement supersedes any other
agreements or representations, oral or otherwise, express or implied, with
respect to the subject matter hereof which have been made by either party
including the Immunex Corporation Leadership Continuing Policy. The validity,
interpretation, construction and performance of this Agreement shall be governed
by the laws of the State of Washington. All references to sections of the
Exchange Act or the Code shall be deemed also to refer to any successor
provisions to such sections. Any payments provided for hereunder shall be paid
net of any applicable withholding required under federal, state or local law and
any additional withholding to which the Executive has agreed. The obligations of
the Company and the Executive under this Agreement which by their nature may
require either partial or total performance after the expiration of the Term
(including, without limitation, those under Sections 6 and 7 hereof) shall
survive such expiration.

<PAGE>

                  12.  Validity.  The invalidity or unenforceability of any
                       --------
provision of this Agreement shall not affect the validity or enforceability of
any other provision of this Agreement, which shall remain in full force and
effect.

                  13.  Counterparts.  This Agreement may be executed in several
                       ------------
counterparts, each of which shall be deemed to be an original but all of which
together will constitute one and the same instrument.

                  14.  Settlement of Disputes. All claims by the Executive for
                       ----------------------
benefits under this Agreement shall be directed to and determined by the
Committee and shall be in writing. Any denial by the Committee of a claim for
benefits under this Agreement shall be delivered to the Executive in writing and
shall set forth the specific reasons for the denial and the specific provisions
of this Agreement relied upon. The Committee shall afford a reasonable
opportunity to the Executive for a review of the decision denying a claim and
shall further allow the Executive to appeal to the Committee a decision of the
Committee within sixty (60) days after notification by the Committee that the
Executive's claim has been denied. Notwithstanding the above, in the event of
any dispute, any decision by the Committee hereunder shall be subject to a de
novo review by a court or, if the Executive is a party to an arbitration
agreement with the Company, by an arbitrator. Notwithstanding any provision of
this Agreement to the contrary, the Executive shall be entitled to seek specific
performance of the Executive's right to be paid until the Date of Termination
during the pendency of any dispute or controversy arising under or in connection
with this Agreement.

                  15.  Definitions.  For purposes of this Agreement, the
                       -----------
following terms shall have the meanings indicated below:

                  (A)  "Affiliate" shall have the meaning set forth in Rule
12b-2 promulgated under Section 12 of the Exchange Act.

                  (B)  "Auditor" shall have the meaning set forth in Section 6.2
hereof.

                  (C)  "Base Amount" shall have the meaning set forth in section
280G(b)(3) of the Code.

                  (D)  "Beneficial Owner" shall have the meaning set forth in
Rule 13d-3 under the Exchange Act.

                  (E)  "Board" shall mean the Board of Directors of the Company.

                  (F)  "Cause" for termination by the Company of the Executive's
employment shall mean (i) the willful and continued failure by the Executive to
substantially perform the Executive's duties with the Company (other than any
such failure resulting from the Executive's incapacity due to physical or mental
illness or any such actual or anticipated failure after the issuance of a Notice
of Termination for Good Reason by the Executive pursuant to Section 7.1 hereof)
that has not been cured within 30 days after a written demand for substantial
performance is delivered to the Executive by the Committee, which demand
specifically identifies the manner in which the Committee believes that the
Executive has not substantially performed the Executive's duties, or (ii) the
willful engaging by the Executive in conduct which is demonstrably and
materially injurious to the Company or its subsidiaries, monetarily or
otherwise. For purposes of clauses (i) and (ii) of this definition, (x) no act,
or failure to act, on the Executive's part shall be deemed "willful" unless
done, or omitted to be done, by the Executive not in good faith and without
reasonable belief that the Executive's act, or failure to act, was in the best
interest of the Company and (y) in the event of a dispute concerning the
application of this provision, no

<PAGE>

claim by the Company that Cause exists shall be given effect unless the Company
establishes to the Committee by clear and convincing evidence that Cause exists.

                  (G)  A "Change in Control" shall be deemed to have occurred if
the event set forth in any one of the following paragraphs shall have occurred:

                               (I)    any Person, other than American Home
                  Products Corporation, a Delaware corporation, or any of its
                  Affiliates, is or becomes the Beneficial Owner, directly or
                  indirectly, of securities of the Company representing 35% or
                  more of the combined voting power of the Company's then
                  outstanding securities, excluding any Person who becomes such
                  a Beneficial Owner in connection with a transaction described
                  in clause (i) of paragraph (III) below; or

                               (II)   the following individuals cease for any
                  reason to constitute a majority of the number of directors
                  then serving: individuals who, as of the date hereof,
                  constitute the Board and any new director (other than a
                  director whose initial assumption of office is in connection
                  with an actual or threatened election contest, including but
                  not limited to a consent solicitation, relating to the
                  election of directors of the Company) (i) whose appointment or
                  election by the Board or nomination for election by the
                  Company's shareholders was approved or recommended by a vote
                  of at least two-thirds (2/3) of the directors then still in
                  office who either were directors on the date hereof or whose
                  appointment, election or nomination for election was
                  previously so approved or recommended or (ii) who is an
                  Investor Director (as defined in the Amended and Restated
                  Governance Agreement, dated as ofDecember 15, 1992, among
                  Lederle Oncology Corporation, American Cyanamid Company and
                  the Company) nominated by American Home Products Corporation
                  or any of its Affiliates pursuant to the terms of the
                  Governance Agreement; or

                               (III)  there is consummated a merger or
                  consolidation of the Company or any direct or indirect
                  subsidiary of the Company with any other Person, other than
                  (i) a merger or consolidation which would result in the voting
                  securities of the Company outstanding immediately prior to
                  such merger or consolidation continuing to represent (either
                  by remaining outstanding or by being converted into voting
                  securities of the surviving entity or any parent thereto) at
                  least 60% of the combined voting power of the securities of
                  the Company or such surviving entity or any parent thereof
                  outstanding immediately after such merger or consolidation, or
                  (ii) a merger or consolidation effected to implement a
                  recapitalization of the Company (or similar transaction) in
                  which no Person (other than American Home Products Corporation
                  and its Affiliates) is or becomes the Beneficial Owner,
                  directly or indirectly, of securities of the Company (not
                  including in the securities Beneficially Owned by such Person
                  any securities acquired directly from the Company or its
                  Affiliates) representing 35% or more of the combined voting
                  power of the Company's then outstanding securities; or

                               (IV)   the shareholders of the Company approve a
                  plan of complete liquidation or dissolution of the Company or
                  there is consummated an agreement for the sale or disposition
                  by the Company of all or substantially all of the Company's
                  assets, other thana sale or disposition by the Company of all
                  or substantially all of the Company's assets to an entity, at
                  least 60% of the combined voting power of the voting
                  securities of which are owned by shareholders of the Company
                  in substantially the same proportions as their ownership of
                  the Company immediately prior to such sale; or

                               (V)    American Home Products Corporation
                  (together with its Affiliates) is or becomes the Beneficial
                  Owner, directly or indirectly, of securities of the

<PAGE>

                  Company (not including in the securities beneficially owned by
                  such Person any securities acquired directly from the Company
                  or its Affiliates) representing 70% or more of the combined
                  voting power of the Company's then outstanding securities.
                  Notwithstanding the foregoing, a "Change in Control" shall not
                  be deemed to have occurred by virtue of the consummation of
                  any transaction or series of integrated transactions
                  immediately following which the record holders of the common
                  stock of the Company immediately prior to such transaction or
                  series of transactions continue to have substantially the same
                  proportionate ownership in an entity which owns all or
                  substantially all of the assets of the Company immediately
                  following such transaction or series of transactions.

                  (H)  "Code" shall mean the Internal Revenue Code of 1986, as
amended from time to time.

                  (I)  "Committee" shall mean the Compensation Committee of the
Board.

                  (J)  "Company" shall mean Immunex Corporation and, except in
determining under Section 15(G) hereof whether or not any Change in Control of
the Company has occurred, shall include any successor to its business and/or
assets which assumes and agrees to perform this Agreement by operation of law,
or otherwise.

                  (K)  "DC Pension Plan" shall mean any tax-qualified,
supplemental or excess defined contribution plan maintained by the Company and
any other defined contribution plan or agreement entered into between the
Executive and the Company.

                  (L)  "Date of Termination" shall have the meaning set forth in
Section 7.2 hereof.

                  (M)  "Disability" shall be deemed the reason for the
termination by the Company of the Executive's employment, if, as a result of the
Executive's incapacity due to physical or mental illness, the Executive shall
have been absent from the full-time performance of the Executive's duties with
the Company for a period of six (6) consecutive months, the Company shall have
given the Executive a Notice of Termination for Disability, and, within thirty
(30) days after such Notice of Termination is given, the Executive shall not
have returned to the full-time performance of the Executive's duties.

                  (N)  "Exchange Act" shall mean the Securities Exchange Act of
1934, as amended from time to time.

                  (O)  "Excise Tax" shall mean any excise tax imposed under
section 4999 of the Code.

                  (P)  "Executive" shall mean the individual named in the first
paragraph of this Agreement.

                  (Q)  "Good Reason" for termination by the Executive of the
Executive's employment shall mean the occurrence, on or after the date of a
Change in Control and without the affected Executive's express written consent
which specifically references this Agreement, of (i) the assignment to the
Executive of duties in the aggregate that are inconsistent with the Executive's
level of responsibility immediately prior to the date of the Change in Control
or any diminution in the nature or status of the Executive's responsibilities
from those in effect immediately prior to the date of the Change in Control
(including, without limitation, the Executive ceasing to be an executive officer
of a public company); (ii) a reduction by the Company in the Executive's annual
base salary, annual incentive compensation opportunity, or long term incentive
compensation opportunity (including an adverse change in performance criteria or
a decrease in the target amount of annual or long term incentive compensation)
from that in effect immediately prior to the Change in Control; or (iii) the
relocation of

<PAGE>

the Executive's principal place of employment or principal place of performance
of Executive's duties to a location more than fifty (50) miles from the
Executive's principal place of employment immediately prior to the date of the
Change in Control. Notwithstanding the preceding sentence, any termination of
employment by the Executive, whether voluntary or involuntary, for any reason or
no reason, after the first anniversary of a Change in Control but within sixty
(60) days following such anniversary shall be deemed to constitute a termination
for Good Reason hereunder, provided that, for purposes of this sentence only, in
determining whether a Change in Control has occurred pursuant to Section 15(G),
any securities acquired by any Person directly from the Company shall not be
included in the securities beneficially owned by such Person.

                  For purposes of any determination regarding the existence of
Good Reason, any claim by the Executive that Good Reason exists shall be
presumed to be correct unless the Company establishes to the Committee by clear
and convincing evidence that Good Reason does not exist.

                  (R)  "Gross-Up Payment" shall have the meaning set forth in
Section 6.2 hereof.

                  (S)  "Notice of Termination" shall have the meaning set forth
in Section 7.1 hereof.

                  (T)  "Person" shall have the meaning given in Section 3(a)(9)
of the Exchange Act as modified and used in Sections 13(d) and 14(d) thereof,
except that such term shall not include (a) the Company or any of its Affiliates
(other than American Home Products Corporation or any of its Affiliates), (b) a
trustee or other fiduciary holding securities under an employee benefit plan of
the Company or any of its subsidiaries, (c) an underwriter temporarily holding
securities pursuant to an offering of such securities or (d) a corporation
owned, directly or indirectly, by the stockholders of the Company in
substantially the same proportions as their ownership of stock of the Company.

                  (U)  "Potential Change in Control" shall be deemed to have
occurred if the event set forth in any one of the following paragraphs shall
have occurred:

                               (I)   the Company enters into an agreement, the
                  consummation of which would result in the occurrence of a
                  Change in Control;

                               (II)  the Company or any Person publicly
                  announces an intention to take or to consider taking actions
                  which, if consummated, would constitute a Change in Control;

                               (III) any Person, other than American Home
                  Products Corporation or any of its Affiliates, becomes the
                  Beneficial Owner, directly or indirectly, of securities of the
                  Company representing 15% or more of either the then
                  outstanding shares of common stock of the Company or the
                  combined voting power of the Company's then outstanding
                  securities; or

                               (IV)  the Board adopts a resolution to the effect
                  that a Potential Change in Control has occurred.

                  (V)  "Retention Payment" shall have the meaning set forth in
the Immunex Corporation Retention Plan.

                  (W)  "Retirement" shall be deemed the reason for the
termination by the Executive of the Executive's employment if such employment is
terminated in accordance with the Company's retirement policy, including early
retirement, generally applicable to its salaried employees.

                  (X)  "Severance Payments" shall have the meaning set forth in
Section 6.1 hereof.

                  (Y)  "Tax Counsel" shall have the meaning set forth in Section
6.2 hereof.

<PAGE>

                  (Z)  "Term" shall mean the period of time described in Section
2 hereof (including any extension, continuation or termination described
therein).

                  (AA) "Total Payments" shall mean those payments so described
in Section 6.2 hereof.

                  IN WITNESS WHEREOF, the parties have executed this Agreement
as of the date first above written.

                                            IMMUNEX CORPORATION

                                            By:________________________________

                                               Name:

                                               Title:

                                            ___________________________________

                                                        Executive

                                            Address:

                                            ___________________________________

                                            ___________________________________

                                            ___________________________________

                                            (Please print carefully)

<PAGE>

                                                                       EXHIBIT A

                     WAIVER AND RELEASE OF CLAIMS AGREEMENT
                     --------------------------------------

                  YOU HAVE BEEN ADVISED TO CONSULT AN ATTORNEY PRIOR TO SIGNING
THIS AGREEMENT.

                  YOU HAVE [FORTY-FIVE] [TWENTY-ONE] DAYS AFTER RECEIVING THIS
AGREEMENT TO CONSIDER WHETHER TO SIGN IT.

                  AFTER SIGNING THIS AGREEMENT, YOU HAVE ANOTHER SEVEN DAYS IN
WHICH TO REVOKE IT, AND IT DOES NOT TAKE EFFECT UNTIL THOSE SEVEN DAYS HAVE
ENDED.

                  In consideration of, and subject to, the payments to be made
to me by Immunex Corporation ("Immunex") or any of its subsidiaries, pursuant to
the Severance Agreement between me and Immunex Corporation dated as of December
16, 2001 (the "Agreement"), which I acknowledge that I would not otherwise be
entitled to receive, I hereby waive any claims I may have for employment or
re-employment by Immunex or any subsidiary or parent of Immunex after the date
hereof, and I further agree to and do release and forever discharge Immunex or
any subsidiary or parent of Immunex and their respective past and present
officers, directors, shareholders, employees and agents from any and all claims
and causes of action, known or unknown, arising out of or relating to my
employment with Immunex or any subsidiary or parent of Immunex or the
termination thereof, including, but not limited to, wrongful discharge, breach
of contract, tort, fraud, the Civil Rights Acts, Age Discrimination in
Employment Act, Employee Retirement Income Security Act, Americans with
Disabilities Act, or any other federal, state or local legislation or common law
relating to employment or discrimination in employment or otherwise.

                  Notwithstanding the foregoing or any other provision hereof,
nothing in this Waiver and Release of Claims Agreement shall adversely affect
(i) my rights under the Agreement and under the Immunex Corporation Retention
Plan; (ii) my rights to benefits other than severance benefits under plans,
programs and arrangements of Immunex or any subsidiary or parent of Immunex; or
(iii) my rights to indemnification under any indemnification agreement,
applicable law and the certificates of incorporation and bylaws of Immunex and
any subsidiary or parent of Immunex, and my rights under any director's and
officer's liability insurance policy covering me.

                  I acknowledge that I have signed this Waiver and Release of
Claims Agreement voluntarily, knowingly, of my own free will and without
reservation or duress, and that no promises or representations, written or oral,
have been made to me by any person to induce me to do so other than the promise
of payment set

<PAGE>

forth in the first paragraph above and Immunex's acknowledgment of my rights
reserved under the second paragraph above.

                  I understand that this release will be deemed to be an
application for benefits under the Agreement, and that my entitlement thereto
shall be governed by the terms and conditions of the Agreement, and I expressly
hereby consent to such terms and conditions.

                  I acknowledge that I have been given not less than [forty-five
(45)] [twenty-one (21)] days to review and consider this Waiver and Release of
Claims Agreement, and that I have had the opportunity to consult with an
attorney or other advisor of my choice and have been advised by Immunex to do so
if I choose. I may revoke this Waiver and Release of Claims Agreement seven days
or less after its execution by providing written notice to Immunex.

                  Finally, I acknowledge that I have carefully read this Waiver
and Release of Claims Agreement and understand all of its terms. This is the
entire agreement between the parties and is legally binding and enforceable.

                  This Waiver and Release of Claims Agreement shall be governed
and interpreted under federal law and the laws of Washington.

                  I knowingly and voluntarily sign this Waiver and Release of
Claims Agreement.

         Executive:                         IMMUNEX CORPORATION

                                        By:     ______________________

                                       Title:  ______________________

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