IMMUNEX CORP /DE/
DEF 14A, 2000-03-24
BIOLOGICAL PRODUCTS, (NO DIAGNOSTIC SUBSTANCES)
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<PAGE>
                            SCHEDULE 14A INFORMATION

                  Proxy Statement Pursuant to Section 14(a) of
            the Securities Exchange Act of 1934 (Amendment No.    )

    Filed by the Registrant /X/
    Filed by a party other than the Registrant / /

    Check the appropriate box:
    / /  Preliminary Proxy Statement
    / /  Confidential, for Use of the Commission Only (as permitted by Rule
         14a-6(e)(2))
    /X/  Definitive Proxy Statement
    / /  Definitive Additional Materials
    / /  Soliciting Material Pursuant to Section 240.14a-11(c) or Section
         240.14a-12

                              IMMUNEX CORPORATION
- --------------------------------------------------------------------------------
                (Name of Registrant as Specified In Its Charter)

- --------------------------------------------------------------------------------
    (Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

/X/  No fee required

/ /  Fee computed on table below per Exchange Act Rules 14a-6(i)(1)
     and 0-11

    (1) Title of each class of securities to which transaction applies:

        ------------------------------------------------------------------------
    (2) Aggregate number of securities to which transaction applies:

        ------------------------------------------------------------------------
    (3) Per unit price or other underlying value of transaction computed
        pursuant to Exchange Act Rule 0-11 (set forth the amount on which the
        filing fee is calculated and state how it was determined):

        ------------------------------------------------------------------------
    (4) Proposed maximum aggregate value of transaction:

        ------------------------------------------------------------------------
    (5) Total fee paid:

        ------------------------------------------------------------------------

/ / Fee paid previously with preliminary materials.

/ / Check box if any part of the fee is offset as provided by Exchange Act Rule
    0-11(a)(2) and identify the filing for which the offsetting fee was paid
    previously. Identify the previous filing by registration statement number,
    or the Form or Schedule and the date of its filing.

    (1) Amount Previously Paid:

        ------------------------------------------------------------------------
    (2) Form, Schedule or Registration Statement No.:

        ------------------------------------------------------------------------
    (3) Filing Party:

        ------------------------------------------------------------------------
    (4) Date Filed:

        ------------------------------------------------------------------------

<PAGE>
                                     [LOGO]

March 24, 2000

Dear Immunex Shareholders:

    I am pleased to invite you to our annual meeting of shareholders. The
meeting will be at 9:00 a.m. on Tuesday, April 25, 2000 at Benaroya Hall,
Illsley Ball Nordstrom Recital Hall, 200 University Street, Seattle, Washington.

    At the meeting, you will elect nine directors to our Board of Directors,
vote upon a proposal to ratify the Board's selection and appointment of Ernst &
Young LLP to serve as the independent auditors to Immunex and transact any
business properly presented at the meeting. You also will have the opportunity
to hear what has happened in our business in the past year and to ask questions.
You will find other detailed information about Immunex and our operations,
including our audited financial statements, in the enclosed 1999 annual report
to shareholders.

    We hope you can join us on April 25. Whether or not you can attend, please
read the enclosed proxy statement. When you have done so, please mark your votes
on the enclosed proxy, sign and date the proxy, and return it to us in the
enclosed envelope. Your vote is important, so please return your proxy promptly.

                                          Sincerely,

                                          Scott G. Hallquist
                                          SECRETARY
<PAGE>
                                     [LOGO]

                              51 UNIVERSITY STREET
                           SEATTLE, WASHINGTON 98101

                                                                  March 24, 2000

                    NOTICE OF ANNUAL MEETING OF SHAREHOLDERS
                       TO BE HELD TUESDAY, APRIL 25, 2000

                            ------------------------

Dear Immunex Shareholders:

    On Tuesday, April 25, 2000, Immunex Corporation will hold its annual meeting
of shareholders at Benaroya Hall, Illsley Ball Nordstrom Recital Hall, 200
University Street, Seattle, Washington. The annual meeting will begin at
9:00 a.m. Only shareholders who owned stock at the close of business on the
record date, March 8, 2000, can vote at this meeting or any adjournments that
may take place. At the annual meeting, we will ask you to:

    - Elect a Board of Directors;

    - Ratify the Board's selection and appointment of Ernst & Young LLP to serve
      as the independent auditors to Immunex; and

    - Transact any other business properly presented at the meeting.

    YOUR BOARD OF DIRECTORS RECOMMENDS THAT YOU VOTE IN FAVOR OF THE TWO
PROPOSALS OUTLINED IN THIS PROXY STATEMENT.

    At the meeting, we will also report on our 1999 business results and other
matters of interest to shareholders.

    To assure your representation at the annual meeting, you are urged to
complete, sign, date, and return the enclosed proxy card as soon as possible in
the enclosed postage prepaid envelope. Your stock will be voted in accordance
with the instructions you give on your proxy card. You may, of course, attend
the annual meeting and vote in person even if you have previously returned your
proxy card.

    The approximate date of mailing for this proxy statement and accompanying
proxy card(s) is March 24, 2000.

                                          BY ORDER OF THE BOARD OF DIRECTORS,

                                          Scott G. Hallquist
                                          SECRETARY

  PLEASE NOTE THAT ATTENDANCE AT THE ANNUAL MEETING WILL BE LIMITED TO
SHAREHOLDERS AS OF THE RECORD DATE, OR THEIR AUTHORIZED REPRESENTATIVES, AND
GUESTS OF IMMUNEX. YOUR ADMITTANCE TICKET IS INCLUDED IN THIS MAILING.
<PAGE>
                                    CONTENTS

<TABLE>
<S>                                                           <C>
INFORMATION CONCERNING SOLICITATION AND VOTING..............      1
  General...................................................      1
  Record Date and Outstanding Shares........................      1
  Quorum....................................................      1
  Revocability of Proxies...................................      1
  Solicitation of Proxies...................................      1
  Voting....................................................      1
PRINCIPAL SHAREHOLDERS......................................      3
SECURITY OWNERSHIP OF MANAGEMENT............................      4
PROPOSAL 1: ELECTION OF DIRECTORS...........................      5
  Information on Committees of the Board of Directors and
    Meetings................................................      7
  Compensation of Directors.................................      8
PROPOSAL 2: RATIFICATION OF THE INDEPENDENT AUDITORS........      9
RELATIONSHIP WITH AMERICAN HOME PRODUCTS CORPORATION AND
  AMERICAN CYANAMID COMPANY.................................      9
  Background................................................      9
  Governance Agreement......................................      9
    Overview................................................      9
    Designation of Candidates for Our Board.................     10
    Certain Approval Rights.................................     11
    Subscription Rights of American Home Products...........     12
    Transfer Of Our Common Stock By American Home
     Products...............................................     12
    Material Transactions With American Home Products.......     13
    Registration Rights.....................................     13
    Termination.............................................     13
  Product Rights Agreement..................................     13
  TACE Agreement............................................     15
  TNFR License and Development Agreement....................     15
  ENBREL Promotion Agreement................................     15
  Convertible Subordinated Note.............................     16
EXECUTIVE OFFICERS..........................................     17
EXECUTIVE COMPENSATION......................................     18
  Summary Compensation Table................................     18
  Option Grants in Fiscal 1999..............................     19
  Option Exercises in Fiscal 1999 and Year-End Values.......     19
  Change in Control and Employment Agreements...............     20
  Compensation Committee Interlocks and Insider
    Participation...........................................     20
REPORT ON EXECUTIVE COMPENSATION BY THE COMPENSATION
  COMMITTEE AND THE STOCK OPTION PLAN ADMINISTRATION
  COMMITTEE.................................................     20
PERFORMANCE GRAPH...........................................     23
  Comparison of Cumulative Total Return Among Immunex, S&P
    500 Index and Nasdaq Pharmaceutical Index...............     23
SECTION 16(a) BENEFICIAL OWNERSHIP REPORTING COMPLIANCE.....     24
SHAREHOLDER PROPOSALS FOR 2001 ANNUAL MEETING...............     24
OTHER MATTERS...............................................     24
</TABLE>
<PAGE>
                                     [LOGO]

                            ------------------------

                                PROXY STATEMENT

                            ------------------------

                 INFORMATION CONCERNING SOLICITATION AND VOTING

GENERAL

    The Board of Directors of Immunex Corporation is sending you this proxy
statement in connection with its solicitation of proxies for use at Immunex's
2000 annual meeting of shareholders. The annual meeting will be held at Benaroya
Hall, Illsley Ball Nordstrom Recital Hall, 200 University Street, Seattle,
Washington on Tuesday, April 25, 2000 at 9:00 a.m. We intend to give or mail to
shareholders definitive copies of this proxy statement and accompanying proxy
card(s) on or about March 24, 2000.

RECORD DATE AND OUTSTANDING SHARES

    Only those shareholders who owned common stock at the close of business on
March 8, 2000, the record date for the annual meeting, can vote. At that date,
there were 166,608,925 issued and outstanding shares of common stock.

QUORUM

    A quorum for the annual meeting is a majority of the outstanding shares of
common stock entitled to vote and present, whether in person or by proxy, at the
annual meeting.

REVOCABILITY OF PROXIES

    If you give your proxy to us, you have the power to revoke it at any time
before it is exercised. Your proxy may be revoked by:

    - notifying the Secretary of Immunex in writing before the annual meeting;

    - delivering to the Secretary of Immunex before the annual meeting a signed
      proxy with a later date; or

    - attending the annual meeting and voting in person.

SOLICITATION OF PROXIES

    We retained MacKenzie Partners, Inc., 156 Fifth Avenue, New York, New York,
to help solicit proxies. We will pay the cost of their services, which is
estimated at approximately $5,000, plus expenses. Proxies will be solicited by
personal interview, mail and telephone. In addition, we may reimburse brokerage
firms and other persons who represent beneficial owners of common stock for
their expenses in forwarding solicitation materials to beneficial owners.
Proxies may also be solicited by our directors, officers and regular employees,
without additional compensation, personally or by telephone or facsimile.

VOTING

    You are entitled to one vote for each share of common stock you hold. For
the election of directors, the directors who receive the greatest number of
affirmative votes cast by holders of common stock present, in person or by
proxy, and entitled to vote at the annual meeting, will be elected to the Board.
You are not entitled to cumulate votes in the election of directors. To ratify
the Board's selection and appointment of Ernst & Young LLP to serve as the
independent auditors to Immunex, the affirmative
<PAGE>
vote of the holders of a majority of common stock present, in person or by
proxy, and entitled to vote at the annual meeting, is required.

    If your shares are represented by proxy, they will be voted in accordance
with your directions. If your proxy is signed and returned without any direction
given, your shares will be voted in accordance with our recommendation. We are
not aware, as of the date of this proxy statement, of any matters to be voted on
at the annual meeting other than as stated in the proxy statement and the
accompanying notice of annual meeting of shareholders. If any other matters are
properly brought before the annual meeting, the enclosed proxy gives
discretionary authority to the persons named in it to vote the shares in their
best judgment.

    Abstention and broker non-votes will have no effect on either of the
proposals to be voted on at the annual meeting since they will not represent
votes cast at the annual meeting for the purpose of voting on such proposals.

                                       2
<PAGE>
                             PRINCIPAL SHAREHOLDERS

    The following table sets forth as of December 31, 1999, information
regarding all shareholders known by Immunex to be the beneficial owners of more
than 5% of our outstanding voting securities, based on publicly available
information. Where applicable, this information is adjusted to reflect the
2-for-1 splits of our common stock effected March 25, 1999 and August 26, 1999
and the 3-for-1 split of our common stock effected March 20, 2000.

<TABLE>
<CAPTION>
NAME AND ADDRESS OF                                                AMOUNT AND NATURE OF   PERCENT OF
BENEFICIAL OWNER                                  TITLE OF CLASS   BENEFICIAL OWNERSHIP     CLASS
- -------------------                               --------------   --------------------   ----------
<S>                                               <C>              <C>                    <C>
American Home Products Corporation..............   Common Stock        281,820,093(1)        55.3%
  Five Giralda Farms
  Madison, New Jersey 07940
FMR Corp........................................   Common Stock         56,140,710(2)        11.4%
  82 Devonshire Street
  Boston, Massachusetts 02109
Wellington Management Company, LLP..............   Common Stock         30,168,825(3)         6.1%
  75 State Street
  Boston, Massachusetts 02109
</TABLE>

- ------------------------

(1) In a filing on Schedule 13D/A, dated August 6, 1999, American Home Products
    Corporation, or AHP, reported sole voting and dispositive power over all of
    the shares that it beneficially owns. These shares include shares held by
    two wholly owned subsidiaries of AHP and 15,544,041 shares underlying a
    convertible subordinated note held by AHP, as described below under
    RELATIONSHIP WITH AMERICAN HOME PRODUCTS CORPORATION AND AMERICAN CYANAMID
    COMPANY--CONVERTIBLE SUBORDINATED NOTE.

(2) In a filing on Schedule 13G/A, dated February 14, 2000, FMR Corp. reported
    sole voting power with respect to 6,716,730 shares and sole dispositive
    power with respect to 56,140,710 shares. Fidelity Management & Research
    Company, or Fidelity, a wholly-owned subsidiary of FMR Corp., is the
    beneficial owner of 48,914,880 shares. Edward C. Johnson 3d (Chairman of FMR
    Corp.), FMR Corp. (through its control of Fidelity) and the funds each has
    sole power to dispose of 48,914,880 shares owned by the funds. Neither FMR
    Corp. nor Edward C. Johnson 3d has sole power to vote or direct the voting
    of shares owned directly by the funds. Fidelity Management Trust Company, or
    FMTC, a wholly-owned subsidiary of FMR Corp., is the beneficial owner of
    5,738,400 shares. Edward C. Johnson 3d and FMR Corp. (through its control of
    FMTC), each has sole dispositive power over 5,738,400 shares and sole power
    to vote or to direct the voting of 5,229,300 shares and no power to vote or
    direct the voting of 509,100 shares owned by certain institutional accounts.
    Edward C. Johnson 3d owns 12.0% of the outstanding voting stock of FMR Corp.
    and Abigail Johnson, a director of FMR Corp., owns 24.5% of the outstanding
    voting stock of FMR Corp. Members of the Edward C. Johnson 3d family, which
    collectively control approximately 49% of the voting power of FMR Corp., may
    be deemed, under the Investment Company Act of 1940, to form a "controlling
    group" with respect to FMR Corp. The foregoing information is based solely
    on a review of the referenced Schedule 13G/A.

(3) In a filing on Schedule 13G/A, dated February 9, 2000, Wellington Management
    Company, LLP, or WMC, reported that WMC, in its capacity as investment
    advisor, may be deemed to beneficially own 30,168,825 shares which are held
    of record by clients of WMC. WMC further reported shared voting power with
    respect to 10,510,125 shares and shared dispositive power with respect to
    29,888,925 shares. WMC reported sole voting or dispositive power over no
    shares.

                                       3
<PAGE>
                        SECURITY OWNERSHIP OF MANAGEMENT

    The following table sets forth as of March 8, 2000, the number of our
outstanding voting securities beneficially owned by (1) each director and each
director nominee, (2) each executive officer for whom compensation is reported
in this proxy statement, and (3) all current directors and executive officers as
a group. Where applicable, this information is adjusted to reflect the 2-for-1
splits of our common stock effected March 25, 1999 and August 26, 1999 and the
3-for-1 split of our common stock effected March 20, 2000.

<TABLE>
<CAPTION>
                                                                   AMOUNT AND NATURE OF   PERCENT OF
NAME OF BENEFICIAL OWNER                          TITLE OF CLASS   BENEFICIAL OWNERSHIP     CLASS
- ------------------------                          --------------   --------------------   ----------
<S>                                               <C>              <C>                    <C>
Edward V. Fritzky...............................   Common Stock        2,363,369(1)             *
Scott G. Hallquist..............................   Common Stock          923,592(2)             *
Peggy V. Phillips...............................   Common Stock          826,531(3)             *
Douglas E. Williams.............................   Common Stock          601,200(4)             *
John E. Lyons...................................   Common Stock          228,000(5)             *
David A. Mann...................................   Common Stock           74,760(6)             *
Edith W. Martin.................................   Common Stock           60,000(7)             *
Kirby L. Cramer.................................   Common Stock           48,000(8)             *
Robert I. Levy..................................   Common Stock               --               --
John R. Considine...............................   Common Stock               --               --
Joseph M. Mahady................................   Common Stock               --               --
All current directors and executive officers as
 a group (11 persons)...........................   Common Stock        5,125,452              1.0%
</TABLE>

- ------------------------

*   Less than 1% of the outstanding shares of common stock.

(1) Includes 2,178,000 shares that are issuable upon exercise of stock options
    that are currently exercisable or are exercisable within 60 days.

(2) Includes 655,625 shares that are issuable upon exercise of stock options
    that are currently exercisable or are exercisable within 60 days.

(3) Includes 813,600 shares that are issuable upon exercise of stock options
    that are currently exercisable or are exercisable within 60 days.

(4) Includes 601,200 shares that are issuable upon exercise of stock options
    that are currently exercisable or are exercisable within 60 days.

(5) Includes 228,000 shares that are issuable upon exercise of stock options
    that are currently exercisable or are exercisable within 60 days.

(6) Includes 74,760 shares that are issuable upon exercise of stock options that
    are currently exercisable or are exercisable within 60 days.

(7) Includes 48,000 shares that are issuable upon exercise of stock options that
    are currently exercisable or are exercisable within 60 days.

(8) Includes 48,000 shares that are issuable upon exercise of stock options that
    are currently exercisable or are exercisable within 60 days.

                                       4
<PAGE>
    The following table sets forth as of March 8, 2000, the number of
outstanding voting securities of American Home Products Corporation, or AHP,
beneficially owned by (1) each director and each director nominee, (2) each
executive officer for whom compensation is reported in this proxy statement, and
(3) all current directors and executive officers as a group.

<TABLE>
<CAPTION>
                                                                   AMOUNT AND NATURE OF   PERCENT OF
NAME OF BENEFICIAL OWNER                          TITLE OF CLASS   BENEFICIAL OWNERSHIP     CLASS
- ------------------------                          --------------   --------------------   ----------
<S>                                               <C>              <C>                    <C>
Edward V. Fritzky...............................  Common Stock                 --            --
Scott G. Hallquist..............................  Common Stock              1,500             *
Peggy V. Phillips...............................  Common Stock                 --            --
Douglas E. Williams.............................  Common Stock                 --            --
John E. Lyons...................................  Common Stock                 --            --
David A. Mann...................................  Common Stock                 --            --
Edith W. Martin.................................  Common Stock                 --            --
Kirby L. Cramer.................................  Common Stock                 --            --
Robert I. Levy..................................  Common Stock            424,536(1)          *
John R. Considine...............................  Common Stock            290,494(2)
Joseph M. Mahady................................  Common Stock             73,812(3)          *
All current directors and executive officers as
  a group (11 persons)..........................  Common Stock            790,342             *
</TABLE>

- ------------------------

*   Less than 1% of the outstanding shares of common stock.

(1) Includes 362,198 shares that are issuable upon exercise of stock options
    that are currently exercisable or are exercisable within 60 days.

(2) Includes 241,132 shares that are issuable upon exercise of stock options
    that are currently exercisable or are exercisable within 60 days.

(3) Includes 69,998 shares that are issuable upon exercise of stock options that
    are currently exercisable or are exercisable within 60 days.

                       PROPOSAL 1: ELECTION OF DIRECTORS

    The shareholders will elect a Board of Directors consisting of nine
directors, each of whom will hold office for one year or until their successor
is elected and qualified to serve.

    Under the Amended and Restated Governance Agreement dated as of
December 15, 1992, AHP is entitled to designate three directors, know as AHP
Directors, who can be officers or employees of AHP and one independent director
for election to the Board. While AHP has not yet designated a candidate to serve
as an independent director, it may do so at any time. For more information
regarding this topic, see the section below entitled RELATIONSHIP WITH AMERICAN
HOME PRODUCTS CORPORATION AND AMERICAN CYANAMID COMPANY--GOVERNANCE
AGREEMENT--DESIGNATION OF CANDIDATES FOR OUR BOARD. Proxies may not be voted for
a greater number of persons than the number of nominees named.

    The Board has approved the nominees named below, who were designated in
accordance with the Governance Agreement. Unless otherwise instructed, persons
named in the accompanying proxy will vote for those nominees. Although we
anticipate that all the nominees will be available to serve as directors, should
any of them not accept the nomination, or otherwise be unable to serve, the
proxies will be voted for the election of such substitute nominees in accordance
with the Governance Agreement.

                                       5
<PAGE>
    The following table sets forth the name and age of each nominee for election
as a director, the positions and offices held by the nominee with Immunex and
the period during which the nominee has served as a director of Immunex:

<TABLE>
<CAPTION>
                                                                                            DIRECTOR
       NAME            AGE                  POSITIONS AND OFFICES WITH IMMUNEX               SINCE
- -------------------  --------   ----------------------------------------------------------  --------
<S>                  <C>        <C>                                                         <C>
Edward V. Fritzky       49      Chief Executive Officer; President; Chairman of the Board     1994
John R. Considine       49      Director                                                      2000
Kirby L. Cramer         63      Director                                                      1987
Robert I. Levy          62      Director                                                      1998
John E. Lyons           74      Director                                                      1993
Joseph M. Mahady        46      Director                                                      1998
Edith W. Martin         54      Director                                                      1993
Peggy V. Phillips       46      Executive Vice President; Chief Operating Officer;            1996
                                Director
Douglas E. Williams     41      Executive Vice President; Chief Technology Officer;           1996
                                Director
</TABLE>

    Mr. Fritzky has been Chief Executive Officer and Chairman of the Board since
January 1994. In April 1999, Mr. Fritzky was named President. Mr. Fritzky served
as President of Lederle Laboratories, a division of American Cyanamid Company,
from 1992 to 1994 and as Vice President of Lederle from 1989 to 1992. Prior to
joining Lederle, Mr. Fritzky was an executive of Searle Pharmaceuticals, Inc., a
subsidiary of Monsanto Company. During his tenure at Searle, Mr. Fritzky was
Vice President, Marketing and later President and General Manager of Searle
Canada, Inc. and Lorex Pharmaceuticals, a joint venture company. Mr. Fritzky
also serves on the board of directors of Geron Corporation and SonoSite, Inc.
Mr. Fritzky is the Chair of the Succession Planning Committee.

    Mr. Considine has been Senior Vice President, Finance of AHP since
January 2000. He has held various positions with AHP since 1983. Mr. Considine
held the positions of Vice President-Finance from 1992 to 2000, Vice President
and Treasurer from 1989 to 1992, and Vice President and Comptroller from 1985 to
1989. For a discussion of AHP's right to designate Mr. Considine as our
director, see the section below entitled RELATIONSHIP WITH AMERICAN HOME
PRODUCTS CORPORATION AND AMERICAN CYANAMID COMPANY.

    Mr. Cramer has been a director since October 1987. Mr. Cramer is Chairman of
SonoSite, Inc. and Northwestern Trust and Investors Advisory Company and
Chairman Emeritus of Hazleton Laboratories Corporation. He also serves on the
board of directors of DJ Orthopedics, LLC, Landec Corporation, Huntington Life
Science, Ragen MacKenzie Group, Incorporated and The Commerce Bank of
Washington. Mr. Cramer is the Chair of the Compensation Committee and the Stock
Option Plan Administration Committee.

    Dr. Levy has been a director since April 1998. Dr. Levy has served since
March 1998 as Senior Vice President, Science and Technology of AHP. From
March 1992 until March 1998, he served as the President of the Wyeth-Ayerst
Research division of AHP. For a discussion of AHP's right to designate Dr. Levy
as our director, see the section below entitled RELATIONSHIP WITH AMERICAN HOME
PRODUCTS CORPORATION AND AMERICAN CYANAMID COMPANY.

    Mr. Lyons has been a director since June 1993. Mr. Lyons retired as Vice
Chairman of the Board of Merck & Company in 1991. He joined Merck in 1950 as a
Research Chemist and held a number of senior marketing and sales positions in
the Merck, Sharp & Dohme division of Merck & Company, serving as its President
from 1975 to 1985. He was appointed Corporate Senior Vice President of Merck in
1982, Executive Vice President in 1985, and Vice Chairman of the Board in 1988.
Mr. Lyons also serves on the board of directors of Synaptic Pharmaceutical
Company.

    Mr. Mahady has been a director since February 1998. Mr. Mahady has held
various positions with AHP and its Wyeth-Ayerst Laboratories division since
1979. He has been the President of

                                       6
<PAGE>
Wyeth-Ayerst North America since September 1997. Previously, he was President of
Wyeth-Ayerst Pharmaceutical Business Division from August 1995 through
September 1997, having previously been promoted to Senior Vice President in
February 1995 and Vice President in October 1991. For a discussion of AHP's
right to designate Mr. Mahady as our director, see the section below entitled
RELATIONSHIP WITH AMERICAN HOME PRODUCTS CORPORATION AND AMERICAN CYANAMID
COMPANY.

    Dr. Martin has been a director since June 1993. Dr. Martin is Vice President
and Chief Information Officer of Halliburton Company, a position she has held
since March 1999. Dr. Martin was Vice President and Chief Information Officer of
Eastman Kodak Company, a position she held from January 1996 to December 1997.
Between September 1994 and February 1996, Dr. Martin was the Executive Vice
President and Chief Technology Officer of the Student Loan Marketing
Association, or Sallie Mae. Prior to joining Sallie Mae, Dr. Martin had been
Vice President and Chief Information Officer of the International
Telecommunications Satellite Organization, or INTELSAT, since 1992. Prior to
joining INTELSAT, Dr. Martin was Vice President, High Technology Center, The
Boeing Company. Dr. Martin also serves on the board of directors of Information
Resources, Inc. and Pharmacopeia, Inc. Dr. Martin is the Chair of the Audit
Committee.

    Ms. Phillips has been a director since July 1996. She joined Immunex in 1986
and was named Senior Vice President, Pharmaceutical Development in
September 1994. In October 1999, Ms. Phillips was named Executive Vice President
and Chief Operating Officer. She was elected an executive officer of Immunex in
July 1995. From 1991 until its dissolution in January 1995, Ms. Phillips was
Senior Vice President and Chief Operating Officer of Immunex Research and
Development Corporation, Immunex's former wholly owned research and development
corporation. Ms. Phillips received an M.S. in microbiology from the University
of Idaho.

    Dr. Williams has been a director since April 1996. He joined Immunex in 1988
and served as Vice President of Research and Development from 1992 until
September 1994, when he was appointed Senior Vice President, Discovery Research.
Dr. Williams was named Executive Vice President and Chief Technology Officer in
October 1999. He was elected an executive officer of Immunex in July 1995.
Dr. Williams received a Ph.D. in physiology from the State University of New
York at Buffalo, Roswell Park Memorial Institute Division.

INFORMATION ON COMMITTEES OF THE BOARD OF DIRECTORS AND MEETINGS

    During the last fiscal year there were six meetings of the Board. All
incumbent directors except Dr. Levy attended at least 75% of the Board meetings
held. All incumbent directors attended at least 75% of the meetings held by all
committees on which they served.

    In accordance with the Governance Agreement, the Board maintains an Audit
Committee, a Compensation Committee, a Nominating Committee, a Stock Option Plan
Administration Committee and a Succession Planning Committee.

    The Audit Committee, currently composed of Messrs. Cramer and Lyons and
Dr. Martin, is responsible, among other things, for recommending the selection
of certified public accountants to the Board, reviewing the scope and results of
the audits and reviewing our accounting policies and procedures, and system of
internal controls. During the past year, there were three Audit Committee
meetings.

    The Compensation Committee, currently composed of Messrs. Considine, Cramer
and Lyons and Dr. Martin, is responsible for, among other things, recommending
to the Board the adoption and amendment of employee benefit plans and
arrangements and the engagement of, and terms of any employment agreements and
arrangements with, and termination of, all corporate executive officers. During
the past year, there was one Compensation Committee meeting.

                                       7
<PAGE>
    The Nominating Committee, currently composed of Mr. Considine and
Dr. Williams, is responsible for the nomination of directors and the
solicitation of shareholder proxies. Under the Governance Agreement, designation
of directors for nomination is to be made exclusively by Immunex and AHP. During
the past year, there were no Nominating Committee meetings.

    The Stock Option Plan Administration Committee, currently composed of
Messrs. Considine, Cramer, Lyons and Mahady and Drs. Levy and Martin, is
responsible, among other things, for administering all of our stock option
plans. During the past year, there were four Stock Option Plan Administration
Committee meetings.

    The Succession Planning Committee, currently composed of Messrs. Considine,
Cramer and Fritzky, is responsible for planning for the succession of officers
and for aiding in the identification and development of qualified candidates for
successors to our current executive officers and in key leadership positions
throughout Immunex. During the past year, there were two Succession Planning
Committee meetings.

COMPENSATION OF DIRECTORS

    Each independent director receives $6,000 per quarter and, if the
independent director is a chairperson of a committee, an additional $1,000 per
quarter. In addition, each independent director receives $1,000 for each Board
meeting and each committee meeting attended in person and $500 for each meeting
attended telephonically. Directors designated for election by our management,
know as management directors, and AHP directors receive no additional
compensation for attending Board or committee meetings.

    Under our Stock Option Plan for Nonemployee Directors, each independent
director receives a one-time grant of an option to purchase 10,000 shares of
common stock on the day such director is initially elected or appointed to the
Board. Each continuing independent director receives an annual grant of an
option to purchase 5,000 shares of common stock immediately following each
year's annual meeting of shareholders. However, if a grant to an independent
director occurs within 90 days following certain changes in our capitalization
(for example, a stock split), the shares subject to such grant are
proportionately adjusted to be the same number and class of shares that would
have been subject to such grant had it been outstanding immediately prior to the
change in capitalization. In the case of the 1999 grant to independent
directors, the 2-for-1 split of our common stock effected March 25, 1999
resulted in an application of this adjustment provision and a grant of 10,000
shares to the continuing independent directors on April 29, 1999. All such
options vest at a rate of 20% per year over a five-year period.

                                       8
<PAGE>
              PROPOSAL 2: RATIFICATION OF THE INDEPENDENT AUDITORS

    The Board unanimously recommends that you vote "FOR" ratification of their
selection and appointment of Ernst & Young LLP, certified public accountants, to
serve as our independent auditors for the year ending December 31, 2000. On
February 17, 2000, the Board unanimously approved the recommendation of the
Audit Committee that Ernst & Young LLP be retained as our independent auditors
and selected and appointed Ernst & Young LLP to serve as our independent
auditors for the year ending December 31, 2000. The Board's selection and
appointment is subject to reconsideration by the Board in the event that our
shareholders fail to ratify the selection of Ernst & Young LLP pursuant to this
Proposal 2. The Board believes that Ernst & Young LLP, having been our auditor
since Immunex's inception, has the advantage of a longstanding, constructive
relationship with Immunex.

    If the shareholders fail to approve this Proposal 2, the Board will
reconsider whether or not to retain Ernst & Young LLP as our independent
auditors. Whether or not the Board's selection is ratified, the Board in its
discretion may retain Ernst & Young LLP or direct the appointment of a different
independent accounting firm at any time during the year if the Board determines
that such action would be in the best interests of Immunex or our shareholders.

    A representative of Ernst & Young LLP is expected to be present at the
annual meeting, with the opportunity to make a statement, if the representative
so desires, and is expected to be available to respond to appropriate questions
from shareholders.

            RELATIONSHIP WITH AMERICAN HOME PRODUCTS CORPORATION AND
                           AMERICAN CYANAMID COMPANY

BACKGROUND

    In 1993, the predecessor to the current Immunex Corporation was merged into
Lederle Oncology Corporation, a subsidiary of American Cyanamid Company, or
Cyanamid, created for the purpose of merging Cyanamid's Lederle Laboratories
oncology business in the United States and Canada with our biopharmaceutical
business. As a result of the merger, Cyanamid received the number of shares
equal to 53.5% of our common stock and dilutive securities outstanding
immediately following the merger. In late 1994, AHP purchased all of the common
stock of Cyanamid. Thus, AHP owns Cyanamid's and its affiliate's interest in our
common stock. Before AHP's purchase of Cyanamid, we entered into an agreement
with AHP under which AHP agreed to protect our rights under our agreements with
Cyanamid and be bound by Cyanamid's obligations under these agreements. AHP or
various divisions or affiliates of AHP have assumed some of the rights and
obligations of Cyanamid under the various agreements that we entered into with
Cyanamid at or after the time of the merger, including various supply, license
and distribution agreements. In the following discussion, AHP refers to AHP, or
its various divisions or affiliates, including Cyanamid.

    Immunex and AHP are parties to numerous agreements that AHP assumed from
Cyanamid or that Immunex entered into directly with AHP. The following
agreements, in particular the Governance Agreement, together with the Product
Rights Agreement, establish the framework for our ongoing relationship with AHP.

GOVERNANCE AGREEMENT

    OVERVIEW

    At the same time that we entered into the 1993 merger, we entered into an
Amended and Restated Governance Agreement with Cyanamid. AHP assumed the rights
and obligations of Cyanamid under

                                       9
<PAGE>
the Governance Agreement, which includes, among other matters, agreements
relating to the following:

    - the corporate governance of Immunex, including the composition of our
      Board of Directors;

    - rights of AHP to purchase additional shares of our common stock from us if
      specified events occur;

    - future purchases and sales of our common stock by AHP;

    - the right of members of our Board designated by AHP to approve specified
      corporate actions;

    - the requirement that a supermajority of the members of our Board approve
      specified corporate actions; and

    - payments to be made by AHP to us in the event that the products acquired
      under the 1993 merger did not achieve net sales targets. This provision
      expired on December 31, 1997 and AHP made its final payment under this
      provision to us in February 1998.

    DESIGNATION OF CANDIDATES FOR OUR BOARD

    Our Board, following the annual meeting, will consist of nine directors.
Under the Governance Agreement, three directors are designated for election by
our management, three are designated for election by AHP and three independent
directors are designated for election by agreement of Immunex and AHP. AHP has
the right to designate a fourth independent director for election, but has not
exercised this right.

    At all times during the term of the Governance Agreement, the number of
directors that AHP and our management have the right to designate will be
determined by the percentage interest of Immunex beneficially owned by AHP. If
AHP's interest is:

    - below 20%, AHP will have no right to designate any directors, and our
      management will have the right to designate six management directors;

    - 20% or above but less than 35%, AHP will have the right to designate one
      AHP director, and our management will have the right to designate five
      management directors;

    - 35% or above but less than 45%, AHP will have the right to designate two
      AHP directors, and our management will have the right to designate four
      management directors;

    - 45% or above but less than 65%, AHP will have the right to designate three
      AHP directors, and our management will have the right to designate three
      management directors; and

    - 65% or above, AHP will have the right to designate four AHP directors, by
      adding an additional AHP director to the Board, and our management will
      have the right to designate three management directors.

In the event that AHP's interest is such that there are more AHP directors on
the Board than AHP has the right to designate, AHP will promptly cause to
resign, and take all other action reasonably necessary to cause the prompt
removal of, that number of AHP directors as required to make the remaining
number of AHP directors conform with the above formula. Similarly, in the event
that AHP's interest is such that there are more management directors on the
Board than our management has the right to designate, our management will
promptly cause to resign, and take all other action reasonably necessary to
cause the prompt removal of, that number of management directors as required to
make the remaining number of management directors conform with the above
formula.

    With certain exceptions, AHP and the management directors will have the
right to designate replacements for directors designated under the Governance
Agreement by AHP or the management

                                       10
<PAGE>
directors, at the termination of such director's term or upon death,
resignation, retirement, disqualification, removal from office or other cause.
The Board will elect each person so designated upon nomination by the Nominating
Committee, which consists of an equal number of directors designated by each of
Immunex and AHP. No individual who is an officer, director, partner or principal
shareholder of any competitor of Immunex or any of its subsidiaries (other than
AHP and its affiliates) may be designated to serve as our director.

    In any election of directors or any meeting of our shareholders called
expressly for the removal of directors, AHP and its affiliates will vote their
shares of common stock for all nominees in proportion to the votes cast by our
other shareholders, except that AHP and its affiliates may cast any or all of
their votes, in their sole discretion, (1) in favor of any nominee designated by
AHP under the Governance Agreement and (2) in connection with any election
contest to which Rule 14a-11 under the Securities Exchange Act of 1934, as
amended, applies. With certain limited exceptions, in all other matters
submitted to a vote of our shareholders, AHP may vote any or all of its shares
in its sole discretion.

    CERTAIN APPROVAL RIGHTS

    So long as AHP has the right to designate at least two AHP directors, the
approval of at least one of the AHP directors will be required for the Board to
approve and authorize certain corporate actions. Such actions include, without
limitation:

    - the entry by us or any of our subsidiaries into any merger or
      consolidation or the acquisition by us or any of our subsidiaries of any
      business or assets that would constitute more than 10% of the fair market
      value of the total assets of Immunex and our subsidiaries;

    - the sale, lease, pledge, grant of security interest in, license, transfer
      or other disposal by us or any of our subsidiaries of more than 10% of the
      fair market value of the total assets of Immunex and our subsidiaries;

    - with specified exceptions, the issuance of any debt or equity securities
      or other capital stock of Immunex or any of our subsidiaries;

    - subject to specified exceptions, a reclassification, split, redemption or
      other acquisition of any of our debt or equity securities of Immunex or
      any of our subsidiaries;

    - any amendment to our Articles of Incorporation or Bylaws or any change in
      the size or composition of the Board or a committee thereof, except in
      accordance with the Governance Agreement;

    - the establishment of any committee of the Board not specifically described
      in the Governance Agreement;

    - any change in our accounting policies or procedures or those of any of our
      subsidiaries;

    - the payment or discharge of any claim, liability or obligation other than
      in the ordinary course of business, except where such claim, liability or
      obligation does not exceed $350,000;

    - the commencement or termination of any suit, litigation or proceeding with
      respect to patent rights, and any other suit, litigation or proceeding
      that involves a claim, liability or obligation in excess of $350,000 or
      that is material to our business or assets;

    - any (a) incurrence of indebtedness for borrowed money other than as
      provided for in our annual operating plans provided to AHP or its
      affiliates from time to time or (b) capital expenditure by us or any of
      our subsidiaries that is greater than both (1) $350,000 and (2) the amount
      provided for such expenditure in our annual operating plans;

                                       11
<PAGE>
    - the institution by us or any of our subsidiaries of any shareholder rights
      plan or similar plan or device;

    - the acquisition by us or any of our subsidiaries of technology or products
      under any license or similar arrangement if the payments under all such
      licenses that are not contingent on sales of licensed technology or
      products would exceed $500,000 during any year; or

    - the dissolution of Immunex or any of our subsidiaries, the adoption of a
      plan of liquidation for Immunex or any of our subsidiaries or any action
      by us or any of our subsidiaries to commence any bankruptcy or similar
      proceeding.

    The approval of seven directors (or, if the Board consists of more than nine
persons, that number of directors that represents 70% of the total number of
directors, rounded up), will be required for the Board to approve any of the
following:

    - the employment of our senior officers, specifically our Chief Executive
      Officer, Chief Operating Officer, Chief Financial Officer and Chief
      Scientific Officer;

    - annual operating plans for Immunex and our subsidiaries, which shall
      include all material capital expenditures and borrowing plans applicable
      to the year in question;

    - our five-year product development and facility plans; and

    - amendment of the Governance Agreement or provisions of our Articles of
      Incorporation or Bylaws implementing the provisions of the Governance
      Agreement (this act also requires the additional approval of two
      independent directors).

    The approval of six directors, which six directors must include each of the
independent directors, will be required to authorize and approve the termination
of any senior officer.

    SUBSCRIPTION RIGHTS OF AMERICAN HOME PRODUCTS

    So long as AHP has the right to designate at least one AHP director, AHP
must be offered the right to purchase a pro rata share of new securities prior
to any issuance of securities by us. The foregoing right does not apply,
however, to securities issued upon exercise of outstanding options or warrants
and to other issuances specified in the Governance Agreement.

    So long as AHP has the right to designate at least one AHP director, AHP has
the option to purchase from us on a quarterly basis additional shares of our
common stock or other voting stock to the extent necessary to permit AHP to
maintain the percentage of shares of our common stock or other voting stock, as
the case may be, owned by AHP and its affiliates as of the immediately preceding
quarter. The per share purchase price of these shares of our common stock or
other voting stock will be equal to the fair market value of such shares, as
determined in accordance with the Governance Agreement, on the date of AHP's
purchase.

    TRANSFER OF OUR COMMON STOCK BY AMERICAN HOME PRODUCTS

    AHP and its wholly owned subsidiaries may transfer all, but not less than
all, of the shares of our common stock beneficially owned by them to any other
person other than an affiliate of AHP, provided that such other person has
offered to acquire all of our outstanding shares of common stock on the same
terms and conditions as those offered to AHP. In addition, if AHP intends to
transfer its shares of our common stock, AHP is required to notify us of that
intent and, for three months after such notice, we will have the opportunity to
present to AHP a potential buyer willing to purchase all, but not less than all,
of the shares of our common stock beneficially owned by AHP and its wholly owned
subsidiaries. In the event that a potential buyer is presented, AHP may not
consummate a sale on terms less favorable to AHP than those proposed by the
potential buyer.

                                       12
<PAGE>
    MATERIAL TRANSACTIONS WITH AMERICAN HOME PRODUCTS

    We may not enter into any contract, agreement or transaction with AHP or any
of its affiliates that is material to our business, taken as a whole, unless
two-thirds of the members of the Board, excluding the AHP directors and
including at least two independent directors, approve that contract, agreement
or transaction.

    REGISTRATION RIGHTS

    Under the Governance Agreement, a group of our securities, all of which are
currently beneficially owned by AHP, are referred to as Registrable Securities.
The Registrable Securities include the securities issued to Cyanamid as part of
the 1993 merger, any securities issued pursuant to the Governance Agreement and,
at AHP's option, the securities issuable upon conversion of a convertible
subordinated note held by AHP, which is described below under the caption
CONVERTIBLE SUBORDINATED NOTE. The holders of at least 25% of the Registrable
Securities may request that we file a registration statement under the
Securities Act of 1933, as amended, covering the registration of any or all
Registrable Securities held by those holders. We are not obligated to effect
more than three such registrations. The Governance Agreement, however, does not
limit the number of short-form registrations on Form S-3 that may be requested
and obtained if we are eligible to use Form S-3, as long as the estimated
aggregate offering price to the public exceeds $25 million and the other
provisions of the Governance Agreement are satisfied.

    Subject to specified conditions, if we propose to file a registration
statement under the Securities Act on any form (other than on Form S-4 or S-8)
that also would permit the registration of Registrable Securities, and such
filing is for the general registration of shares of common stock for cash, we
must give notice thereof to the holders of the registration rights and permit
those holders to include Registrable Securities in the registration.

    AHP's registration rights are subject to conditions set forth in the
Governance Agreement. In addition, the Governance Agreement sets forth specific
procedures relating to those registration rights and detailed obligations of the
parties relating thereto. All expenses incident to the performance by us of our
obligations relating to the registration of AHP's shares of common stock will be
paid by us, except that the holders exercising registration rights will pay all
expenses incident to the second or third registration. In addition, the holders
of Registrable Securities will pay the underwriting commissions and discounts
applicable to securities offered for their account in connection with any
registrations, filings and qualifications made pursuant to the Governance
Agreement, as well as related attorneys' fees. Immunex and the holders of
Registrable Securities each have agreed to indemnify the other, in certain
instances, for liabilities incurred in connection with such registrations.

    TERMINATION

    The Governance Agreement will terminate at the earlier of (1) when AHP and
its affiliates beneficially own 95% of all classes and series of common stock
and (2) when AHP and its affiliates no longer own any such shares.

PRODUCT RIGHTS AGREEMENT

    In July 1998, we entered into a Product Rights Agreement with AHP, under
which we granted AHP an option to obtain exclusive, royalty-bearing worldwide
licenses to a limited number of our products for all clinical indications. This
option is referred to as a Product Call. Under the Product Rights Agreement, AHP
also owns a right of first refusal to our covered products and technologies that
may only be exercised if our Board decides that Immunex will not market a
covered product or technology by itself in any part of the world where Immunex
has or acquires marketing rights. AHP's right of first refusal, which is subject
to specified negotiation periods and establishment of mutually

                                       13
<PAGE>
acceptable terms, applies to our covered products and technologies in all
fields, including NUVANCE-TM- (IL-4 receptor) and TNF related apoptosis inducing
ligand (TRAIL), but not including LEUKINE-Registered Trademark- (sargramostim,
GM-CSF), MOBISTA-TM- (Flt3 ligand) (formerly known as MOBIST-TM-), AVREND-TM-
(CD40 ligand), IL-15, and several other Immunex products.

    The Product Rights Agreement provides AHP with a Product Call for up to four
Immunex products over a period discussed below. The Product Rights Agreement
also provides that AHP must exercise a Product Call for an Immunex product
within specified time periods determined by reference to when Immunex formally
accords a product status as an IND-track product and when the first positive
Phase II clinical data for that Immunex product is available, or else AHP will
lose the right to use a Product Call on that Immunex product. Some Immunex
products are excluded from AHP's Product Calls, including
ENBREL-Registered Trademark- (etanercept), NUVANCE, LEUKINE, MOBISTA, AVREND,
IL-15, any product marketed by Immunex as of July 1, 1998, and several other
products. In August 1999, Immunex provided notice to AHP that Immunex had
accorded IND-track status to TRAIL, thereby commencing the time period during
which AHP may exercise a Product Call with respect to our interest in TRAIL.

    If AHP exercises a Product Call for an Immunex product, AHP and Immunex will
enter into an elected product agreement granting AHP exclusive worldwide rights
(or if less than exclusive worldwide rights are held by us, all of our rights)
to this Immunex product for all indications. Under the elected product
agreement, AHP will pay us an initial fee, milestone payments and royalties on
any future worldwide net sales of this Immunex product after regulatory
approvals. The initial fee, milestone payments and royalties are determined by
the development stage of the product when AHP exercises the Product Call. In
total, the initial fees and milestone payments range from $25 million if we have
given the product IND status, up to $70 million if we have given notice to AHP
that data from the first positive Phase II clinical trial results are available
for the product. The royalties AHP pays to us increase based on the development
stage of the product and based upon the product attaining specified annual net
sales thresholds.

    Under the Product Rights Agreement, we have the right to keep ownership to
up to two of our products for which AHP has exercised Product Calls, referred to
as a Conversion Right, in exchange for our commitment to pay milestone payments
and royalties to AHP and, in the case of the second Conversion Right only, an
initial fee. Our milestone payments to AHP are fixed at one-half the amount AHP
would otherwise pay us for a Product Call, and our royalties payable to AHP are
always fixed at the lowest of the four levels of royalties that AHP would
otherwise pay us after exercising a Product Call. If we exercise one of our
Conversion Rights for an Immunex product, which must be exercised within
30 days after AHP exercises one of its Product Calls, we will enter into a
converted product agreement with AHP for the product that will provide for
Immunex payments to AHP as discussed above, unless AHP has exercised its option
to obtain a replacement Product Call, as discussed below. We may not exercise
our Conversion Rights on both of the first two Product Calls exercised by AHP.
If we exercise a Conversion Right, AHP may within 30 days elect to obtain one
replacement Product Call from Immunex. If AHP makes this election, AHP waives
its right to receive any applicable initial fee, milestone payments and
royalties from us on this converted product. If either party exercises its
rights under the Product Rights Agreement and acquires or retains rights to an
Immunex product, the company that exercised these rights assumes independent
development responsibility for that product, including the payment of all costs
for future product development.

    The Product Rights Agreement terminated several prior research and
product-related agreements between Immunex and AHP, except for a right of first
refusal to our products and technologies previously held by AHP under one of the
terminated agreements. As a result of terminating these agreements, our oncology
research payment obligations to AHP were canceled, and our exclusive rights in
the United States and Canada to specified new AHP oncology products were
terminated. Under another terminated agreement, AHP returned its exclusive
ownership rights outside the United States and Canada to specified new Immunex
oncology products in exchange for royalty

                                       14
<PAGE>
payments from Immunex to AHP on future sales of LEUKINE, MOBISTA, and IL-15
outside the United States and Canada. These products are not currently approved
or sold outside the United States and Canada. The Product Rights Agreement also
terminated an agreement between Immunex and AHP for the development and
commercialization of MOBISTA.

    AHP's rights to exercise Product Calls under the Product Rights Agreement
terminate upon the first to occur of the following events:

    - AHP has exercised Product Calls and entered into elected product
      agreements for four of our products, subject to our two Conversion Rights
      and AHP's replacement Product Call;

    - June 30, 2008, with an additional year if we exercise both of our
      Conversion Rights; or

    - upon the later of June 30, 2003 or the date by which AHP has been given a
      total of eight opportunities to exercise a Product Call, except that this
      number increases to nine opportunities in specified circumstances.

    AHP's right of first refusal to our covered products and technologies
terminate upon the later of

    - June 30, 2003 or

    - the date that AHP or its affiliates no longer own a majority of our common
      stock.

TACE AGREEMENT

    In December 1995, we entered into research and license agreements with AHP
relating to tumor necrosis factor alpha converting enzyme. Pursuant to these
TACE Agreements, Immunex has granted AHP a worldwide exclusive license under
Immunex intellectual property relating to TACE, and agreed to collaborate with
AHP in developing TACE inhibitors, in consideration of specified fixed payments
for research services, and contingent additional payments that are payable upon
achievement of specified research and clinical milestone events. In 1999,
Immunex recognized $1.6 million in revenues under the TACE Agreements. In
September 1997, in conjunction with the Promotion Agreement discussed below, AHP
and Immunex amended one of the TACE Agreements in order to substantially
increase the royalty payable by AHP to us on the first TACE molecule approved by
the United States Food and Drug Administration, if any.

TNFR LICENSE AND DEVELOPMENT AGREEMENT

    In July 1996, we entered into a TNFR License and Development Agreement, or
TNFR Agreement, with AHP, under which we retained marketing rights to ENBREL in
the United States and Canada, and AHP retained marketing rights to ENBREL
outside of the United States and Canada. The TNFR Agreement also addresses joint
project management, cost sharing for development activities related to ENBREL,
manufacturing responsibilities, intellectual property protection and disposition
of rights upon relinquishment or termination of product development. Previously,
AHP's rights to ENBREL outside the United States and Canada had been stated in a
superseded research and development agreement between Immunex and Cyanamid.
AHP's shared development costs totaled $23,986,000 during 1999.

ENBREL PROMOTION AGREEMENT

    In September 1997, we entered into an ENBREL Promotion Agreement with AHP,
under which AHP, acting through its Wyeth-Ayerst Laboratories division, acquired
the rights to promote ENBREL to all appropriate customer segments in the United
States and Canada for all approved indications other than oncology. Under the
terms of the ENBREL Promotion Agreement, AHP may pay Immunex up to $100 million
in nonrefundable scheduled payments for the U.S. and Canadian promotion rights
to ENBREL. We have already earned $85 million of these scheduled payments, as
follows:

    - $15 million under the ENBREL Promotion Agreement, which was earned in
      September 1997;

                                       15
<PAGE>
    - $20 million when our biologics license application for ENBREL for advanced
      RA was accepted for review by the FDA, which was earned in June 1998;

    - $30 million upon FDA approval of ENBREL, which was earned in
      November 1998;

    - $10 million upon first achieving $200 million in net sales for ENBREL in
      the United States and Canada in any rolling 12-month period, which was
      earned in August 1999; and

    - $10 million upon first achieving $400 million in net sales for ENBREL in
      the United States and Canada in any rolling 12-month period, which was
      earned in February 2000.

    AHP may pay us an additional $15 million in scheduled payments under the
ENBREL Promotion Agreement if and when an RA-disease modification claim for
ENBREL is obtained from the FDA. We cannot be certain when, or if, we will
receive the $15 million final scheduled payment.

    The ENBREL Promotion Agreement also addresses:

    - the formation of a joint ENBREL Management Committee;

    - payment to AHP of a certain percentage of any gross profits of ENBREL in
      the United States and Canada;

    - an allocation of each party's commercial expenses associated with
      marketing and sales activities with respect to ENBREL;

    - our retained rights to promote ENBREL in the United States and Canada for
      any approved oncology indications and to co-promote ENBREL in the United
      States and Canada for any approved indications promoted by AHP;

    - AHP's reimbursement of certain clinical and regulatory expenses associated
      with obtaining certain new indications for ENBREL;

    - an allocation of certain intellectual property expenses;

    - certain protections for us in the event AHP markets a product in the
      United States and Canada that is directly competitive with ENBREL; and

    - the payment of certain residual royalties to AHP in the three years
      following completion of AHP's activities under the ENBREL Promotion
      Agreement.

CONVERTIBLE SUBORDINATED NOTE

    On May 20, 1999, we issued a seven-year, three percent coupon, convertible
subordinated note to AHP. The principal amount of the note, which was purchased
by AHP in a private placement transaction, totaled $450 million. The note is
convertible into our common stock at a price of $28.95 per share. The conversion
price was set at a 30% premium over the average of the closing prices of our
common stock for eight trading days up to and including May 19, 1999. After
three years, we can redeem the note, as long as our closing common stock price
for 20 consecutive trading days exceeds or equals $34.74 per share. After four
years, we can redeem the note at any time if our closing common stock price for
20 consecutive trading days exceeds or equals the conversion price. AHP may
convert the note into our common stock at any time, including in response to a
notice of redemption by us. For most purposes, the common stock issuable upon
conversion of this note is, at AHP's option, included in AHP's ownership of our
common stock under the Governance Agreement and the Product Rights Agreement,
even if the note is not converted and for as long as AHP or its affiliates own
this note. The per share prices specified above have been adjusted to reflect
the 2-for-1 split of our common stock effected on August 26, 1999 and the
3-for-1 split of our common stock effected March 20, 2000.

                                       16
<PAGE>
                               EXECUTIVE OFFICERS

    The following persons are executive officers of Immunex who will serve in
the capacities noted until April 25, 2000, or until the election and
qualification of their successors. Each officer named below is expected to be
reelected at the Board meeting to be held on April 25, 2000.

<TABLE>
<CAPTION>
                                                                                            OFFICER
       NAME            AGE                  POSITIONS AND OFFICES WITH IMMUNEX               SINCE
- -------------------  --------   ----------------------------------------------------------  --------
<S>                  <C>        <C>                                                         <C>
Edward V. Fritzky       49      Chief Executive Officer; President                            1994
Scott G. Hallquist      46      Executive Vice President; General Counsel; Secretary          1987
Peggy V. Phillips       46      Executive Vice President; Chief Operating Officer             1995
Douglas E. Williams     41      Executive Vice President; Chief Technology Officer            1995
David A. Mann           40      Senior Vice President; Chief Financial Officer; Treasurer     1999
</TABLE>

    The biographical summaries of Mr. Fritzky, Ms. Phillips and Dr. Williams,
are provided above in the section entitled ELECTION OF DIRECTORS.

    Mr. Hallquist has been Executive Vice President, General Counsel and
Secretary since October 1999. He joined Immunex in June 1986, initially as
Director, Legal Affairs. Mr. Hallquist was elected to serve as Secretary in
May 1987 and as Vice President and General Counsel in January 1989. In
October 1990, he was elected Senior Vice President, General Counsel and
Secretary. Prior to joining Immunex, he was employed by E.I. du Pont de
Nemours & Co. as patent counsel. Mr. Hallquist received an M.B.A. and a J.D.
from the University of North Carolina in 1981.

    Mr. Mann has been Senior Vice President and Chief Financial Officer since
October 1999 and Treasurer since July 1999. He served as Interim Chief Financial
Officer and Vice President, Finance from April 1999 to October 1999. Mr. Mann
joined Immunex in 1995, as Vice President and Controller. Prior to joining
Immunex, he was employed at Fred Hutchinson Cancer Research Center as
Controller, and KPMG Peat Marwick as an auditor. Mr. Mann is on the board of
directors of the Seattle-King County Economic Development Council. Mr. Mann
holds a B.A. in accounting from Western Washington University and an M.B.A. from
the University of Washington.

                                       17
<PAGE>
                             EXECUTIVE COMPENSATION

SUMMARY COMPENSATION TABLE

    The following table sets forth compensation information as to (1) our Chief
Executive Officer and (2) our four other most highly compensated executive
officers for services rendered in all capacities during the fiscal years ended
December 31, 1997, 1998 and 1999. Where applicable, this information is adjusted
to reflect the 2-for-1 splits of our common stock effected March 25, 1999 and
August 26, 1999 and the 3-for-1 split of our common stock effected March 20,
2000.

<TABLE>
<CAPTION>
                                                                        LONG-TERM
                                             ANNUAL COMPENSATION      COMPENSATION
                                            ---------------------   -----------------
                                                                    SHARES UNDERLYING
                                                                         OPTIONS             ALL OTHER
NAME AND PRINCIPAL POSITION        YEAR     SALARY($)   BONUS($)        AWARDS(#)       COMPENSATION($) (1)
- ---------------------------      --------   ---------   ---------   -----------------   -------------------
<S>                              <C>        <C>         <C>         <C>                 <C>
Edward V. Fritzky .............    1999     $567,000    $595,350         540,000              $52,870
  Chief Executive Officer;         1998      525,000     525,000         540,000               40,992
  President                        1997      468,000     409,500         480,000               23,929

Peggy V. Phillips .............    1999      329,185     548,438         240,000               34,902
  Executive Vice President;        1998      269,179     311,025         240,000               25,386
  Chief Operating Officer          1997      228,690     214,397         540,000               12,594

Scott G. Hallquist ............    1999      302,832     224,848         240,000               27,500
  Executive Vice President;        1998      280,392     196,837         240,000               15,236
  General Counsel; Secretary       1997      245,960     150,930         255,000               36,227

Douglas E. Williams ...........    1999      252,432     283,982         240,000               24,090
  Executive Vice President;        1998      238,140     193,489         240,000               20,292
  Chief Technology Officer         1997      226,800     141,750         240,000               13,608

David A. Mann .................
  Senior Vice President;           1999      177,821     190,350         249,600                6,767
  Chief Financial Officer;         1998      127,904      59,617         124,200                6,256
  Treasurer                        1997      117,416      30,382         150,000                5,337
</TABLE>

- ------------------------

(1) Consists of matching contributions to a 401(k) savings plan of $47,714,
    $32,011, $24,983, $22,296, and $5,714, payment of excess life insurance
    premiums of $2,945, $1,618, $1,336, $810, and $375, and payment of long-term
    disability premiums of $2,211, $1,273, $1,181, $984, and $678, for
    Mr. Fritzky, Ms. Phillips, Mr. Hallquist, Dr. Williams and Mr. Mann,
    respectively, in 1999. All dollar amounts are rounded to the nearest whole
    dollar.

                                       18
<PAGE>
OPTION GRANTS IN FISCAL 1999

    The following table sets forth information regarding options granted during
the fiscal year ended December 31, 1999 to our Chief Executive Officer and the
other officers for whom compensation is reported in this proxy statement. Where
applicable, this information is adjusted to reflect the 2-for-1 splits of our
common stock effected March 25, 1999 and August 26, 1999 and the 3-for-1 split
of our common stock effected March 20, 2000.

<TABLE>
<CAPTION>
                                                                                               POTENTIAL REALIZABLE VALUE
                                                      INDIVIDUAL GRANTS                                AT ASSUMED
                                  ----------------------------------------------------------          ANNUAL RATES
                                  NUMBER OF      PERCENT OF                                          OF STOCK PRICE
                                  SECURITIES   TOTAL OPTIONS                                          APPRECIATION
                                  UNDERLYING     GRANTED TO                                        FOR OPTION TERM(3)
                                   OPTIONS      EMPLOYEES IN    EXERCISE PRICE    EXPIRATION   ---------------------------
NAME                              GRANTED(#)    FISCAL YEAR      ($/SHARE)(1)      DATE(2)        5%($)          10%($)
- ----                              ----------   --------------   ---------------   ----------   ------------   ------------
<S>                               <C>          <C>              <C>               <C>          <C>            <C>
Edward V. Fritzky...............   540,000           3%             11.479          2/22/09     3,898,361      9,879,211

Peggy V. Phillips...............   240,000           1%             11.479          2/22/09     1,732,605      4,390,760

Scott G. Hallquist..............   240,000           1%             11.479          2/22/09     1,732,605      4,390,760

Douglas E. Williams.............   240,000           1%             11.479          2/22/09     1,732,605      4,390,760

David A. Mann...................   129,600           1%             11.479          2/22/09       935,607      2,371,011
  and...........................   120,000           1%             16.229          4/28/09     1,224,772      3,103,813
</TABLE>

- ------------------------

(1) The exercise price of the options is equal to the fair market value of the
    underlying common stock on the date of grant.

(2) All options granted in 1999 terminate 10-years from the date of grant.

(3) Future value of current year grants assuming appreciation of 5% and 10% per
    year over the 10-year option period. The actual value realized may be
    greater or less than the potential realizable values set forth in the table.

OPTION EXERCISES IN FISCAL 1999 AND YEAR-END VALUES

    The following table sets forth information for the fiscal year ended
December 31, 1999 regarding options exercised by, and held at year end by, our
Chief Executive Officer and the other officers for whom compensation is reported
in this proxy statement. Where applicable, this information is adjusted to
reflect the 2-for-1 splits of our common stock effected March 25, 1999 and
August 26, 1999 and the 3-for-1 split of our common stock effected March 20,
2000.

<TABLE>
<CAPTION>
                                                        NUMBER OF SECURITIES
                                                             UNDERLYING               VALUE OF UNEXERCISED
                                                         UNEXERCISED OPTIONS          IN-THE-MONEY OPTIONS
                                                       AT FISCAL YEAR-END (#)        AT FISCAL YEAR-END ($)
                             SHARES        VALUE     ---------------------------   ---------------------------
                          ACQUIRED ON    REALIZED
NAME                      EXERCISE (#)      ($)      EXERCISABLE   UNEXERCISABLE   EXERCISABLE   UNEXERCISABLE
- ----                      ------------   ---------   -----------   -------------   -----------   -------------
<S>                       <C>            <C>         <C>           <C>             <C>           <C>
Edward V. Fritzky.......    360,000      7,573,737    1,896,000       1,554,000    $66,117,359    $47,310,745

Peggy V. Phillips.......    144,000      2,268,600      630,600         902,400     21,188,653     22,132,185

Scott G. Hallquist......    442,560      8,020,194      647,280         721,560     22,437,857     22,096,531

Douglas E. Williams.....    111,600      1,708,250      447,000         722,400     15,391,528     22,132,185

David A. Mann...........    120,840      1,639,083       66,000         522,960      2,333,187     14,855,909
</TABLE>

                                       19
<PAGE>
CHANGE IN CONTROL AND EMPLOYMENT AGREEMENTS

    Messrs. Fritzky, Hallquist and Mann and Ms. Phillips and Dr. Williams are
each covered under our leadership continuity policy. Our leadership continuity
policy provides that, if an executive officer's employment is terminated under
specified circumstances within two years of the occurrence of a specified change
in control of Immunex, then that executive officer will be entitled to receive
(1) an amount equal to two years of his or her then-current salary and
then-current annual incentive bonus opportunity assuming the target has been
achieved at the 100% level for the two-year period following termination of
employment, less applicable withholdings required by law; (2) continued
participation in our medical, dental, employee assistance and life insurance
plans for two years following such termination; and (3) outplacement services.

COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION

    The Compensation Committee is currently composed of Messrs. Considine,
Cramer and Lyons and Dr. Martin. Mr. Considine is a Senior Vice President of
AHP. For details on our relationship with AHP, see the section above entitled
RELATIONSHIP WITH AMERICAN HOME PRODUCTS CORPORATION AND AMERICAN CYANAMID
COMPANY.

         REPORT ON EXECUTIVE COMPENSATION BY THE COMPENSATION COMMITTEE
               AND THE STOCK OPTION PLAN ADMINISTRATION COMMITTEE

    Our compensation policy as established by the Board is intended to provide
competitive compensation to all employees, giving consideration to the relative
contribution and performance of each employee on an individual basis. It is our
policy to compensate our executive officers at levels consistent with industry
norms, primarily in the form of base salary, together with incentive bonuses. In
addition, it is our policy to grant stock options to each of our executive
officers to align their interests with shareholder value. The biotechnology
industry is extremely competitive with respect to recruitment and retention of
qualified executives; accordingly, we use independently published surveys of
biotechnology industry compensation levels to ensure that our compensation
practices are comparable to other biotechnology companies.

    Determining the compensation of our executive officers is the responsibility
of the Board, through its Compensation Committee, which has overall
responsibility for our compensation policies for senior management. Our Stock
Option Plan Administration Committee is responsible for administering our stock
option plans. The Compensation Committee makes recommendations to the Board as
to the salaries of, and incentive bonuses awarded to, our Chief Executive
Officer and other executive officers. The Stock Option Plan Administration
Committee determines the number and terms of options granted to our Chief
Executive Officer, other executive officers and all other employees.

    Executive compensation consists of three major components: base salary,
annual incentive bonus and stock options. The determination of the base salaries
of the Chief Executive Officer and other executive officers is based on annual
surveys of similar positions at other biotechnology companies, together with
assessments of individual performance and our achievement of predetermined
operating goals that are established annually by the Board (the goals for 1999
are described below). Relative weights are assigned to the factors used to
determine base salaries for individual executives. Assessments of individual
performance include objective standards and subjective evaluations of the value
of individual executives to Immunex. The surveys employed include some, but not
all, of the companies in the Nasdaq Pharmaceutical Index, one of the indices we
use in the performance graph that appears below.

    The Compensation Committee meets each December to determine the annual
salary component of executive compensation to be paid in the following calendar
year, and the amount of cash incentive bonus compensation to be awarded
executives for performance in the current year. The salaries paid

                                       20
<PAGE>
to executives in 1999 were determined by reference to 1998 compensation survey
data, adjusted upwards for inflation during the term between July 1998 and
December 1998. The survey data considered by the Compensation Committee in
determining executive salaries included salary information provided by 51
biotechnology companies having more than 500 employees, as well as a subgroup of
16 of the largest biotechnology companies. This information was blended with
data from the 1998 Towers Perrin Pharmaceutical Industry Executive Compensation
survey, resulting in a competitive comparison compensation rate. The
Compensation Committee believes that the comparison group, the comparison
subgroup, and the pharmaceutical industry data were representative of industry
norms in late 1998; each group is weighted approximately equally by the
Compensation Committee. The comparison subgroup consisted of Agouron
Pharmaceuticals, Inc., ALZA Corporation, Amgen, Inc., Biogen, Inc.,
Genentech, Inc., Chiron Corporation, Centocor, Inc., Genzyme Corporation, Gilead
Sciences, Inc., MedImmune, Inc., Millennium Pharmaceuticals, Inc., Ortho
Biotech, SmithKline Beecham, and biotechnology affiliates or subsidiaries of
Baxter Biotech Group, Bayer Corporation, and Scios Nova, Inc. In the case of
Mr. Fritzky, the Compensation Committee established a base salary of $567,000
for 1999, which represented approximately 101.8% of the average competitive
comparison compensation rate for chief executive officers. The Compensation
Committee established 1999 base salaries for our other executive officers
ranging from 79.7% to 103.7% of the average competitive comparison compensation
rate for executives performing similar functions.

    From time to time, the Compensation Committee considers survey data in
establishing new annual bonus incentive targets for our executive officers,
including annual bonus information provided for executive officers by a
representative group of selected biotechnology companies, as well as from
proprietary sources obtained by the external compensation consultant. The annual
bonus incentive targets most recently approved by the Compensation Committee for
our executive officers represented approximately the 50(th) percentile from
these sources. The Compensation Committee has decided that annual bonus
incentive awards for our executive officers should be driven by our overall
achievements. Company objectives are established at the beginning of each year
and approved by the Board. On an annual basis, the Compensation Committee
conducts an assessment of our overall performance as measured against our
objectives for the applicable year, and at that time the Compensation Committee
determines the maximum percentage of the annual bonus incentive targets payable
to our executive officers. The annual bonus targets can be increased by the
Compensation Committee if we have exceeded our objectives for the year or
decreased if we have failed to meet our objectives for the year. An annual bonus
award can be modified upward (up to 150% of the annual bonus incentive target)
or downward (to 0%) by the Compensation Committee, depending on the strength of
the individual executive officer's performance.

    At its February 1997 meeting, the Compensation Committee approved the
following annual incentive targets for our executive officers, which, with the
exception of the percentage target for the Executive Vice President and Chief
Operating Officer, continue to be the annual incentive targets used by the
Compensation Committee and, thus, are the starting point for calculating the
1999 bonuses. The Chief Executive Officer, Mr. Fritzky, was eligible for an
annual incentive target of 70% of base salary. In addition, the other officers
for whom compensation is reported in this proxy statement are eligible for an
annual incentive target of 45% of base salary or 50% of base salary. In
July 1998, the Compensation Committee increased the annual incentive target from
50% to 55% for the Executive Vice President and Chief Operating Officer, since
she had taken on significant additional responsibilities as the general manager
for ENBREL. Again in September 1999, the Compensation Committee increased the
annual incentive target from 55% to 65% of base salary and also increased the
annual salary for the Executive Vice President and Chief Operating Officer in
recognition of her promotion to that position. Although based on the annual
incentive target percentages, actual incentive bonuses are subject to
modification based on the individual executive officer's performance and based
on whether we have met our objectives for the year.

                                       21
<PAGE>
    At its December 1999 meeting, the Compensation Committee elected to
establish incentive targets for Mr. Fritzky and the other executive officers for
whom compensation is reported in this proxy statement of 150% of the annual
incentive target amount for which each officer was eligible (i.e., 150% of 70%
of base salary for Mr. Fritzky, 150% of 65% for Ms. Phillips, 150% of 50% for
Dr. Williams, and 150% of 45% for Mr. Hallquist and Mr. Mann). The Compensation
Committee's decision took into account our overall performance against
established 1999 objectives, which had been assigned relative weights by the
Board. All of our 1999 objectives previously established by the Board had been
met or exceeded, including achievement of budgeted goals for revenue growth and
improvement in our net operating results; completion of certain studies of
ENBREL in early rheumatoid arthritis and submission of a related FDA
application; accomplishment of certain manufacturing improvements and goals
relating to ENBREL; initiation of a definitive clinical trial program for ENBREL
in chronic heart failure; identification of a third indication for ENBREL for
development and initiation of a related pilot clinical trial program; submission
of an FDA application for NOVANTRONE-REGISTERED TRADEMARK- (mitoxantrone for
injection concentrate) in secondary progressive multiple sclerosis; advancement
of clinical trial program for MOBISTA in peripheral blood stem cell
mobilization; initiation of certain clinical trials of NUVANCE; initiation of a
Phase II clinical trial of AVREND; and completion of certain pre-clinical
studies. To qualify compensation for deductibility for federal income tax
purposes, it is our policy to meet the requirements for exclusion from the limit
on deduction imposed by Section 162(m) of the Internal Revenue Code, by paying
performance-based compensation if possible and, with respect to cases in which
it is not possible to meet the requirements for exclusion from Section 162(m) of
the Internal Revenue Code, we intend to minimize any award of compensation in
excess of the limit.

    In addition, the Compensation Committee elected to endorse the target annual
bonus award of Mr. Fritzky and increase the annual bonus award of the other
executive officers discussed above based on the strength of their individual
performance in 1999. In the case of Mr. Fritzky as Chief Executive Officer, the
Compensation Committee maintained his annual bonus award at 100% of 70% of his
base salary based on his very strong individual performance in 1999. Thus, when
both adjustments to Mr. Fritzky's annual bonus award are combined (i.e., 150% of
70% of base salary for our overall performance against 1999 objectives, and 100%
of 70% of his base salary based on his 1999 individual performance),
Mr. Fritzky received 150% (150% multiplied by 100%) of 70% of his 1999 base
salary of $567,000, which amounted to an annual bonus award of $595,350.

    Options to purchase shares of our stock were granted to the officers named
in this report, as well as other employees, during 1999. The option grant was
undertaken pursuant to our long-term incentive performance award program,
initially implemented in 1993, wherein employees are eligible to receive a grant
of stock options dependent on individual performance and position held. Under
this program in 1999, Mr. Fritzky received a grant to purchase 540,000 shares of
stock; other officers named in this proxy statement received grants to purchase
between 240,000 and 249,600 shares. The share numbers specified above have been
adjusted to reflect the 2-for-1 splits of our common stock

                                       22
<PAGE>
effected March 25, 1999 and August 26, 1999 and the 3-for-1 split of our common
stock effected March 20, 2000.

       COMPENSATION COMMITTEE

       John R. Considine
       Kirby L. Cramer
       John E. Lyons
       Edith W. Martin

       STOCK OPTION PLAN ADMINISTRATION COMMITTEE
       John R. Considine
       Kirby L. Cramer
       Robert I. Levy
       John E. Lyons
       Joseph M. Mahady
       Edith W. Martin

                               PERFORMANCE GRAPH
                    COMPARISON OF CUMULATIVE TOTAL RETURN(1)
          AMONG IMMUNEX, S&P 500 INDEX AND NASDAQ PHARMACEUTICAL INDEX

EDGAR REPRESENTATION OF DATA POINTS USED IN PRINTED GRAPHIC

<TABLE>
<CAPTION>
            PHARMACEUTICALS  S&P 500  IMMUNEX
<S>         <C>              <C>      <C>
12/30/1994           100.00   100.00   100.00
12/29/1995           154.69   134.11   110.92
12/31/1996           187.88   161.29   131.08
12/31/1997           278.98   211.30   363.00
12/31/1998           407.54   267.65   845.74
12/31/1999           183.24   319.91  2944.34
</TABLE>

- ------------------------

(1) Assumes $100 invested at the close of trading on December 31, 1994 in our
    common stock, in the S&P 500 Index and in the Nasdaq Pharmaceutical Index.

    NOTE: Stock price performance shown above for our common stock is historical
and not necessarily indicative of future price performance.

                                       23
<PAGE>
            SECTION 16(A) BENEFICIAL OWNERSHIP REPORTING COMPLIANCE

    Section 16(a) of the Securities Exchange Act of 1934, as amended, requires
our officers, directors and persons who own more than 10% of a registered class
of our equity securities to file reports of ownership and changes in ownership
with the Securities and Exchange Commission. Officers, directors and
greater-than-10% shareholders are required by Commission regulation to furnish
us with copies of all Section 16(a) forms they file.

    Based solely on our review of the copies of forms we received, or written
representations from certain reporting persons that no forms were required for
those persons, we believe that during 1999 all filing requirements required by
Section 16(a) applicable to our officers, directors and greater-than-10%
beneficial owners were complied with, with the exception of the filing
requirements applicable to Leonard R. Stevens, who failed to file a timely
Form 4 to report his acquisition of our common stock in his Immunex 401(k)
retirement account, and to Peggy V. Phillips, who failed to file a timely
Form 5 to report her gift of our common stock to a charity.

                 SHAREHOLDER PROPOSALS FOR 2001 ANNUAL MEETING

    Under the Securities and Exchange Commission's proxy rules, shareholder
proposals that meet certain conditions may be included in our proxy statement
and form of proxy for a particular annual meeting. Shareholders that intend to
present a proposal at our 2001 annual meeting must give notice of the proposal
to us no later than November 18, 2000 to be considered for inclusion in the
proxy statement and form of proxy relating to that meeting. Shareholders that
intend to present a proposal that will not be included in the proxy statement
and form of proxy must give notice of the proposal to us no fewer than 60 nor
more than 90 days prior to the date of the 2001 annual meeting. Even if we
receive a proposal from a qualified shareholder in a timely manner, it will not
guarantee that proposals inclusion in our proxy materials or its presentation at
the 2001 annual meeting because there are other requirements in the proxy rules.

                                 OTHER MATTERS

    As of the date of this proxy statement, the Board does not intend to
present, and has not been informed that any other person intends to present, any
matters for action at the annual meeting other than the matters specifically
referred to in this proxy statement. If other matters properly come before the
annual meeting, it is intended that the holders of the proxies will act with
respect thereto in accordance with their best judgment.

    Copies of our 1999 annual report to shareholders are being mailed to
shareholders, together with this proxy statement, form of proxy and notice of
annual meeting of shareholders. Additional copies may be obtained from the
Secretary of Immunex, 51 University Street, Seattle, Washington 98101.

    OUR ANNUAL REPORT ON FORM 10-K FOR THE FISCAL YEAR ENDED DECEMBER 31, 1999,
AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION, IS INCLUDED IN OUR 1999
ANNUAL REPORT TO SHAREHOLDERS.

                                          BY ORDER OF THE BOARD OF DIRECTORS,

                                          Scott G. Hallquist

                                          SECRETARY

Seattle, Washington
March 24, 2000

                                       24
<PAGE>

                            IMMUNEX CORPORATION

         THIS PROXY IS SOLICITED BY THE BOARD OF DIRECTORS FOR THE
               ANNUAL MEETING OF SHAREHOLDERS--APRIL 25, 2000

     The undersigned hereby appoint(s) Edward V. Fritzky and Peggy V. Phillips
and each of them as proxies, with full power of substitution, to represent and
vote as designated all shares of Common Stock of Immunex Corporation held of
record by the undersigned on March 8, 2000 at the Annual Meeting of
Shareholders of Immunex to be held at Benaroya Hall, Illsley Ball Nordstrom
Recital Hall, 200 University Street, Seattle, Washington at 9:00 a.m. on
Tuesday, April 25, 2000, with authority to vote upon the following matters and
with discretionary authority as to any other matters that may properly come
before the meeting or any adjournment or postponement thereof.

             IMPORTANT--PLEASE DATE AND SIGN ON THE OTHER SIDE

- --------------------------------------------------------------------------------
                         -  FOLD AND DETACH HERE  -

<PAGE>

      SHARES REPRESENTED BY THIS PROXY WILL BE VOTED AS DIRECTED BY
                 THE SHAREHOLDER IN THE SPACES PROVIDED.
   IF NO DIRECTION IS GIVEN, THIS PROXY WILL BE VOTED "FOR" THE PROPOSALS.

                                                            Please mark
                                                           your votes as    /X/
                                                            indicated in
                                                            this example


The Board of Directors recommends a vote "FOR" the proposals.


                                            FOR all       WITHHOLD AUTHORITY
                                           nominees    to vote for all nominees
1.  Election of the nine nominees            / /                   / /
    to serve as directors until the
    next annual meeting of shareholders
    and until their successors are elected
    and qualify: John R. Considine, Kirby
    L. Cramer, Edward V. Fritzky,
    Robert I. Levy, John E. Lyons,
    Joseph M. Mahady, Edith W. Martin,
    Peggy V. Phillips and Douglas E. Williams.


WITHHOLD for the following only* (Write the name of the nominee(s) in the
space below.)


- --------------------------------------------------------------------------------
*Unless otherwise directed, all votes will be apportioned equally among these
persons for whom authority is given to vote.

                                                   FOR    AGAINST    ABSTAIN
2.  Ratification of the Independent                / /      / /        / /
    Auditors.


PLEASE SIGN EXACTLY AS YOUR NAME APPEARS ON YOUR STOCK CERTIFICATE.
ATTORNEYS, TRUSTEES, EXECUTORS AND OTHER FIDUCIARIES ACTING IN A
REPRESENTATIVE CAPACITY SHOULD SIGN THEIR NAMES AND GIVE THEIR TITLES. AN
AUTHORIZED PERSON SHOULD SIGN ON BEHALF OF CORPORATIONS, PARTNERSHIPS,
ASSOCIATIONS, ETC. AND GIVE HIS OR HER TITLE. IF YOUR SHARES ARE HELD BY TWO
OR MORE PERSONS, EACH PERSON MUST SIGN. RECEIPT OF THE NOTICE OF MEETING AND
PROXY STATEMENT IS HEREBY ACKNOWLEDGED.

Signature(s)                                        Date
            ---------------------------------------      ----------------------
NOTE: Please sign as name appears on your stock certificate. Joint owners should
each sign. When signing as attorney, trustee, executor or other fiduciary acting
in a representative capacity, please give full title as such.

- --------------------------------------------------------------------------------
                         -  FOLD AND DETACH HERE  -


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