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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.