Document:

<PAGE>

                                                                    EXHIBIT 10.3

                               [AHP Letterhead]

                                August 9, 2000

Mr. Edward V. Fritzky
Immunex Corporation
51 University Street
Seattle, Washington 98101

Dear Ed:

          Reference is made to the registration statement on Form S-3 (the
"Shelf Registration Statement") to be filed by Immunex Corporation ("Immunex")
today with the Securities and Exchange Commission (the "Commission") pursuant to
Rule 415 of the Securities Act of 1933, as amended (the "Securities Act"), to
register an aggregate of 70,000,000 shares of common stock, par value $.01 per
share ("Common Stock"), (i) 20,000,000 shares of which shall comprise primary
shares of Common Stock to be sold by Immunex (the "Primary Offering") and (ii)
50,000,000 shares of which shall comprise secondary shares of Common Stock (the
"Secondary Offering" and together with the Primary Offering, the "Offering") to
be sold by American Home Products Corporation ("AHP"). Reference is also made to
the Amended and Restated Governance Agreement, (the "Governance Agreement")
dated as of December 15, 1992, among American Cyanamid Company ("Cyanamid"),
Immunex and Lederle Oncology Corporation, as amended by Amendment No. 1, dated
May 20, 1999, and Amendment No. 2, dated August 8, 2000, each by and between
Cyanamid, Immunex and AHP. This letter shall serve to further clarify our
understanding in connection with AHP's exercise of its registration rights
pursuant to the Governance Agreement. Terms used herein, but not otherwise
defined, shall have the meanings ascribed to them in the Governance Agreement.

          We hereby agree that (i) the filing by Immunex of the Shelf
Registration Statement, (ii) the filing of an amendment or supplement thereto
relating to the Secondary Offering and (iii) its efforts to facilitate the
subsequent sale pursuant to the Secondary Offering, shall be deemed to be in
accordance with Immunex's obligations in connection with AHP's registration
rights pursuant to Section 6.05 of the Governance Agreement.

          The parties further agree that AHP shall determine, after good faith
consultation with Immunex (subject to the provisions of Section 6.03(f) of the
Governance Agreement), the appropriate timing of any sale of shares pursuant to
the Offering; provided, however, notwithstanding anything herein to the
contrary, if the Immunex board of directors determines in its good faith
judgment that it is appropriate to
<PAGE>

Mr. Edward V. Fritzky
Immunex Corporation                                                            2

conduct all or a portion of the Primary Offering, Immunex may proceed with such
Primary Offering, subject to the following conditions:

     (a)  if AHP has not sold shares pursuant to the Secondary Offering, (i)
          that such Primary Offering shall be conducted between March 31, 2001
          and October 31, 2001 and (ii) that, subject to the provisions of
          Section 6.03(e) of the Governance Agreement (as if such provision
          applied to Immunex), the gross proceeds to Immunex from such Primary
          Offering shall not exceed $1 billion; and

     (b)  if AHP has sold shares pursuant to the Secondary Offering, (i) that
          such Primary Offering shall be made on or before March 31, 2001, or
          within four (4) months after the date of such Secondary Offering,
          whichever is later, (ii) that the gross proceeds to Immunex from such
          Primary Offering shall not exceed $1 billion and (iii) that the number
          of shares sold by Immunex pursuant to such Primary Offering shall not
          exceed one half of the number of shares sold by AHP in such Secondary
          Offering;

provided further that, if AHP is not restricted from selling shares of Common
Stock at that time, Immunex shall provide AHP the opportunity to simultaneously
conduct all or a portion of the Secondary Offering and the terms of the
following paragraph shall apply.

          Immunex shall have the right, but not the obligation, to sell such
number of shares which equals no more than one-third of the aggregate size of
the Offering (as may be adjusted as set forth below) but in no event shall the
gross proceeds to Immunex from the Primary Offering exceed $1 billion (inclusive
of any proceeds derived from Immunex's grant to the underwriters of an over-
allotment option). The size of the Offering shall be determined by AHP, after
good faith consultation with Immunex (subject to the provisions of Section
6.03(e) of the Governance Agreement); provided, however, that if the Offering is
expected to generate gross proceeds to Immunex of less than $1 billion, Immunex
shall have the right, subject to the provisions of Section 6.03(e) of the
Governance Agreement (as if such provision applied to Immunex), to increase the
size of the Primary Offering so as to generate expected gross proceeds to it of
no more than $1 billion.

          Pursuant to Section 9.02(a) of the Governance Agreement and to the
extent necessary to permit the Primary Offering on the terms discussed in this
letter, AHP hereby waives, for the periods described in clauses (a)(i) and
(b)(i) of the third paragraph of this letter, its right to have an Investor
Director approve and authorize an issuance of equity securities pursuant to
Section 4.04(c) of the Governance Agreement. In addition, pursuant to Section
9.02(a) of the Governance Agreement, in the event AHP conducts a Secondary
Offering, AHP hereby waives all of its subscription rights related to the
Primary Offering, including both its right to notification and its right to
purchase Immunex Common Stock pursuant to Section 2.01 of the Governance
Agreement.
<PAGE>

Mr. Edward V. Fritzky
Immunex Corporation                                                            3

          AHP agrees that, from the date of the execution of each underwriting
agreement for any Secondary Offering until September 30, 2001, or nine (9)
months after the date of such underwriting agreement, whichever is later, AHP
will not, directly or indirectly, (i) offer, pledge, sell, contract to sell,
sell any option or contract to purchase, grant any option, right or warrant to
purchase, lend or otherwise transfer or dispose of any shares of Common Stock or
any securities convertible into or exchangeable for shares of Common Stock or
(ii) enter into any swap or other arrangement that transfers to another, in
whole or in part, any of the economic consequences of ownership of Common Stock;
whether any transaction described in clause (i) or (ii) of this sentence is to
be settled by delivery of Common Stock or other securities, in cash or
otherwise.

          Immunex hereby agrees to use its best efforts (i) prior to
effectiveness to amend or supplement the Shelf Registration Statement as
required to comply in all material respects with the requirements of the
Securities Act and the Securities Exchange Act of 1934, as amended (the
"Exchange Act"), and the respective rules and regulations thereunder, (ii) to
obtain effectiveness of the Shelf Registration Statement as promptly as
practicable, (iii) to maintain the Shelf Registration Statement continuously
effective for a period of two (2) years and (iv) during the periods in which AHP
is permitted by this letter to sell shares pursuant to the Secondary Offering,
to promptly amend or supplement the Shelf Registration Statement and any
prospectus (the "Shelf Registration Prospectus") relating thereto as necessary
to comply in all material respects with the Securities Act, the Exchange Act and
the respective rules and regulations thereunder. During the periods in which AHP
is permitted by this letter to sell shares pursuant to the Secondary Offering,
Immunex shall advise AHP in writing (a) when the Shelf Registration Statement or
any post-effective amendment thereto shall have become effective; (b) of any
request by the Commission for any amendment or supplement to the Shelf
Registration Statement or the related Shelf Registration Prospectus or for any
additional information; (c) of the issuance by the Commission of any stop order
suspending the effectiveness of the Shelf Registration Statement or the
initiation of any proceedings for that purpose; (d) of the receipt by Immunex of
any notification with respect to the suspension of the qualification of the
Common Stock for sale in any jurisdiction as shall be reasonably appropriate for
the distribution of the Common Stock under the Offering in accordance with
Section 6.06(e) of the Governance Agreement or the initiation or threatening of
any proceeding for such purpose; and (e) of the happening of any event that
requires the making of any changes in the Shelf Registration Statement or
related Shelf Registration Prospectus so that, as of such date, the statements
therein are not misleading and do not omit to state a material fact required to
be stated therein or necessary to make the statements therein (in the case of
the Shelf Registration Prospectus, in the light of the circumstances under which
they were made) not misleading, which notice shall be accompanied by an
instruction to suspend the use of such Shelf Registration Prospectus until the
requisite changes shall have been made.

          Immunex hereby agrees pursuant to Section 6.03(b) of the Governance
Agreement not to withhold approval if AHP selects Morgan Stanley Dean Witter as
co-lead manager and sole book runner of the Offering provided that AHP selects
Merrill Lynch & Co. as co-lead manager for the Offering, and each of Immunex and
AHP hereby
<PAGE>

Mr. Edward V. Fritzky
Immunex Corporation                                                            4

agree that such an underwriting arrangement shall be used for all sales of
shares in any Primary Offering or Secondary Offering. It is hereby agreed that,
for purposes of Section 6.03(e) of the Governance Agreement, Morgan Stanley Dean
Witter shall be deemed the managing underwriter for the Offering.

          AHP hereby agrees that, on or before the consummation of the Financing
(as defined in the Memorandum of Understanding for Helix Project Financing
Guaranty, dated the date hereof), AHP shall convert the $450 million convertible
subordinated note dated May 20, 1999 issued to AHP by Immunex.

          We hereby agree that irreparable damage would occur in the event any
provision of this letter was not performed in accordance with the terms hereof
and that each of us shall be entitled to specific performance of the terms
hereof, in addition to any other remedy at law or equity.  This letter shall be
governed by, and construed in accordance with, the laws of the State of Delaware
without giving effect to principles of conflict of laws.  This letter may be
executed and delivered (including by facsimile transmission) in one or more
counterparts, and by AHP and Immunex in separate counterparts, each of which
when executed and delivered shall be deemed to be an original but all of which
taken together shall constitute one and the same agreement.

          Except as otherwise stated herein, all terms and conditions of the
Governance Agreement shall remain in full force and effect.

                                         AMERICAN HOME PRODUCTS
                                         CORPORATION

                                         By: /s/ Kenneth J. Martin
                                            ----------------------------
                                            Name:  Kenneth J. Martin
                                            Title: Senior Vice President and
                                                   Chief Financial Officer

Accepted and Agreed:
IMMUNEX CORPORATION
Date August 9, 2000

By: /s/ Edward V. Fritzky
   ---------------------------
   Name:   Edward V. Fritzky
   Title:  Chairman of the Board,
           Chief Executive Officer and President<PAGE>

                                                                    EXHIBIT 10.4

                                                                  August 9, 2000

     Re:  Memorandum of Understanding Regarding Greenwich Holding's West
          ------------------------------------- ------------------------
          Greenwich, RI Biopharmaceutical Facility
          -----------------------------------------

Greenwich Holdings Inc. ("Greenwich"), a wholly-owned subsidiary of American
Home Products Corporation ("AHPC"), entered into a definitive Asset Purchase
Agreement (the "Purchase Agreement") with Glaxo Wellcome Biopharmaceuticals Inc.
("GWBI"), a wholly-owned subsidiary of Glaxo Wellcome Inc. ("GW") dated as of
August 23, 1999, to purchase all the real property and biological manufacturing
facility located at 40 Technology Way, West Greenwich, Rhode Island, also known
as the "Wel Gen" facility (the "Facility") from GWBI. On September 24, 1999,
Greenwich consummated the purchase of the Facility pursuant to the terms of the
Purchase Agreement.

Greenwich/AHPC and Immunex Corporation, together with its Affiliates (as defined
below) ("Immunex"), by combining the strengths and competencies of each party,
intend to retrofit "Suites A and D" of the Facility (hereinafter referred to
solely as "Suite A") by mid-2001 to accommodate the commercial manufacture of
ENBREL(R) (etanercept), to perform the initial validation runs for ENBREL and to
commence commercial production of ENBREL. Greenwich/AHPC and Immunex are
exploring the potential of installing equipment and capital improvements in
"Suite B" of the Facility ("Phase A-3") to accommodate future commercial
manufacturing needs for ENBREL. Immunex is exploring the potential of building
additional biotechnology manufacturing capacity on vacant land located at the
Facility outside the footprint of the existing Suites ("Phase B") in a manner
which will not impact the most aggressive accelerated schedule for the retrofit
of Suites A and B (i.e., Phase A-3) of the Facility.

AHPC intends to build a biopharmaceutical manufacturing facility at an affiliate
site of AHPC located in Ireland ("BIOS") for the eventual manufacture of ENBREL,
together with other biotechnology products, and it is anticipated that such site
would be completed in approximately 2004.

Greenwich/AHPC and Immunex have agreed to enter into a Definitive Agreement (as
defined below) regarding the purchase and sale of all of the outstanding capital
stock of Greenwich (the "Greenwich Shares") to Immunex, pursuant to the terms
set forth herein and to be included in the Definitive Agreement. This memorandum
of understanding ("MOU") sets forth the mutual commitments of Greenwich/AHPC and
Immunex with regard to such purchase and sale of the Greenwich Shares, and with
regard to the terms and conditions that will govern the parties' management of
the Facility during the retrofit of Suites A and B (i.e., Phase A-3) of the
Facility.

                                       1
<PAGE>

                                                                  August 9, 2000

1.   PARTIES:            Immunex

                         Greenwich/AHPC

2.   PURCHASE OF         Immunex shall purchase or acquire the Greenwich
     GREENWICH           Shares from Greenwich/AHPC, and Greenwich/AHPC
     SHARES:             shall sell or transfer the Greenwich Shares to
                         Immunex pursuant to the terms and conditions contained
                         in the Definitive Agreement. By owning the Greenwich
                         Shares at the closing of the Definitive Agreement,
                         which shall occur in accordance with the timing for
                         Greenwich/AHPC's transfer of operational control of the
                         Facility to Immunex set forth in Paragraph 4 below (the
                         "Closing"), Immunex will own all right, title and
                         interest in and to the Facility and all improvements
                         thereto since September 24, 1999.

                         Immunex's obligation to purchase the Greenwich Shares
                         is not subject to any financing contingencies.

3.   PURCHASE PRICE:     The purchase price for the Greenwich Shares is sixty
                         million dollars ($60,000,000), plus one hundred percent
                         (100%) of any costs incurred by Greenwich/AHPC related
                         to readying the Facility for approval by the FDA and/or
                         EMEA, including but not limited to capital
                         expenditures, supplies, personnel costs, operating
                         costs (including training), utilities and taxes
                         ("Facility Costs") (together, the "Purchase Price"),
                         payable as follows:

                              (a)  At the signing of the Definitive Agreement
                                   (the "Signing"), Immunex shall pay
                                   Greenwich/AHPC fifty percent (50%) of any
                                   Facility Costs incurred after September 24,
                                   1999 and through the end of the calendar
                                   quarter just prior to the Signing. Within a
                                   reasonable time prior to the Signing,
                                   Greenwich/AHPC shall provide Immunex an
                                   invoice detailing such Facility Costs
                                   incurred

                                       2
<PAGE>

                                                                  August 9, 2000

                                   during such period that will be payable at
                                   the Signing, and as soon as practicable
                                   thereafter, Greenwich/AHPC shall provide
                                   Immunex an invoice detailing any remaining
                                   Facility Costs incurred by Greenwich/AHPC at
                                   the end of the calendar quarter prior to
                                   Signing that had not been reimbursed by
                                   Immunex at the Signing, which invoice will be
                                   payable by Immunex within thirty (30) days.

                              (b)  Immunex shall pay Greenwich/AHPC fifty
                                   percent (50%) of Facility Costs incurred
                                   during each calendar quarter on and after the
                                   Signing within thirty (30) days after
                                   Greenwich/AHPC provides Immunex an invoice
                                   detailing such costs during such calendar
                                   quarter.

                              (c)  At the Closing, Immunex shall pay
                                   Greenwich/AHPC sixty million dollars
                                   ($60,000,000), together with all Facility
                                   Costs incurred on or after the Signing that
                                   have not previously been reimbursed by
                                   Immunex pursuant to Paragraph 3(a) or
                                   Paragraph 3(b) above. Within a reasonable
                                   time prior to the Closing, Greenwich/AHPC
                                   shall provide Immunex an invoice detailing
                                   such costs incurred during such period that
                                   will be payable at the Closing, and as soon
                                   as practicable thereafter, Greenwich/AHPC
                                   shall provide Immunex an invoice detailing
                                   any remaining Facility Costs incurred by
                                   Greenwich/AHPC prior to Closing that had not
                                   been reimbursed by Immunex at the Closing,
                                   which invoice will be payable by Immunex
                                   within thirty (30) days.

                         Immunex shall have the right to audit any invoice
                         provided by Greenwich/AHPC hereunder against original
                         records. Such audit will be allowed within thirty (30)
                         days of an Immunex request.

                         Greenwich/AHPC shall not charge or otherwise pass-
                         through to Immunex any cost of capital for any portion
                         of the Purchase Price.

                                       3
<PAGE>

                                                                  August 9, 2000

4.   TIMING:             The parties shall enter into a mutually acceptable
                         definitive agreement (the "Definitive Agreement"),
                         which the parties shall use diligent efforts to prepare
                         for execution promptly after the execution and delivery
                         of this MOU by all parties. The Definitive Agreement
                         and the Collaboration Agreement contemplated by the
                         Memorandum of Understanding Regarding Allocation of
                         ENBREL Supplies that was entered into the same date as
                         this MOU shall be executed and delivered by the parties
                         simultaneously.

                         Greenwich/AHPC shall sell or transfer all of the
                         Greenwich Shares to Immunex, and transfer operational
                         control of the Facility to Immunex at the Closing,
                         which shall be completed upon the latest to occur of
                         the following: (a) September 30, 2002, (b) such time as
                         at least one hundred (100) kilograms of ENBREL have
                         been successfully manufactured at the Facility, or (c)
                         either the FDA or the EMEA has approved the Facility
                         for commercial production. Successful manufacture of
                         ENBREL means production of bulk drug substance meeting
                         all applicable ENBREL bulk drug substance
                         specifications and FDA regulatory requirements.

                         After the Closing, a reasonable number of AHPC's and
                         its Affiliates' personnel shall be entitled to remain
                         at the Facility as provided for in Paragraph 5 below to
                         facilitate AHPC's and its Affiliates' knowledge and
                         understanding of manufacturing commercial quantities of
                         ENBREL. An "Affiliate" shall mean any corporation or
                         business entity of which a party hereto owns directly
                         or indirectly, fifty percent (50%) or more of the
                         assets or outstanding stock, or any corporation which a
                         party directly or indirectly controls, or any parent
                         corporation that owns, directly or indirectly, fifty
                         percent (50%) or more of the assets or outstanding
                         stock of a party or directly or indirectly controls a
                         party. For purposes of this Agreement, Immunex shall
                         not be deemed to be an Affiliate of Greenwich/AHPC.

                                       4
<PAGE>
                                                                  August 9, 2000

5.   PROJECT SCOPE:      The retrofit of Suites A and B (i.e., Phase A-3) of the
                         Facility is a joint project between Greenwich/AHPC and
                         Immunex managed by Greenwich/AHPC (the "Project"),
                         until the Closing. The Project is a cooperative effort,
                         overseen by the Steering Committee (formed as discussed
                         below), that recognizes and utilizes the strengths and
                         competencies of each party. The Steering Committee
                         shall approve an annual budget for the Facility,
                         including a capital budget. In addition, the Steering
                         Committee shall approve any third party or
                         Greenwich/AHPC capital expenditures associated with the
                         fill and finish of bulk drug substance ENBREL
                         manufactured at the Facility.

                         A six (6)-member Steering Committee shall be formed to
                         oversee the Project during the period of
                         Greenwich/AHPC's ownership of the Greenwich Shares,
                         with Immunex and Greenwich/AHPC each having three (3)
                         representatives. The Steering Committee shall meet on
                         at least a quarterly basis or more frequently as
                         necessary, and decisions of the Steering Committee
                         shall be made by consensus. The Steering Committee
                         shall be disbanded upon Immunex's purchase of the
                         Greenwich Shares.

                         The Steering Committee shall endeavor to reach a
                         consensus on all matters within its purview which are
                         in dispute within a period of ten (10) days after
                         receiving notification that such dispute has been
                         referred to the Steering Committee for resolution. If
                         such a resolution cannot be reached in that time period
                         (or earlier at the election of either party), the
                         matter shall be referred to the Chief Operating Officer
                         of Immunex and the President of Wyeth-Ayerst, or their
                         designees, to resolve in a period of ten (10) days
                         through good faith discussions, or if still unresolved,
                         to endeavor in good faith to promptly agree upon a
                         binding third party dispute resolution mechanism
                         intended to promptly and fairly resolve the matter in
                         dispute.

                                       5
<PAGE>

                                                                  August 9, 2000

                         During the period of Greenwich/AHPC ownership of the
                         Greenwich Shares, and thereafter for such transition
                         period as agreed by the parties (which period shall in
                         no event be more than 24 months), a reasonable number
                         of Immunex and Greenwich/AHPC personnel will be
                         involved in all aspects of start-up and operation of
                         the Facility as are agreed by the parties, and will
                         also have access to all engineering, manufacturing,
                         quality assurance, regulatory, and all other
                         documentation related to the Facility and the ENBREL
                         product(s) manufactured at the Facility, it being
                         understood that during the term of either party's
                         ownership of the Greenwich Shares, and consistent with
                         the terms hereunder, such party will have ultimate
                         decision making authority with respect to day-to-day
                         operations of the Facility.

                         Greenwich/AHPC shall allow Immunex to (a) review and
                         approve each contract involving capital improvements to
                         the Facility which would result in payments greater
                         than three million dollars ($3,000,000) in the
                         aggregate, (b) review and consult with Greenwich/AHPC
                         on all contracts involving the Facility and its
                         operation of duration greater than one (1) year and in
                         an amount greater than two hundred fifty thousand
                         dollars ($250,000) in the aggregate and (c) review and
                         approve each contract involving the fill and finish of
                         ENBREL produced at the Facility. Immunex shall be a
                         party to each such contract under clause (c) above, and
                         to the extent practicable, Greenwich/AHPC shall name
                         Immunex a third party beneficiary under each such
                         contract under clause (a) and (b) above.

6.   IMMUNEX             To the extent that Greenwich/AHPC requests or
     PERSONNEL           otherwise receives technical assistance related to the
     COSTS               Facility from Immunex employees, such technical
                         assistance will be reimbursed at cost by Greenwich/AHPC
                         to Immunex on a quarterly basis. Greenwich/AHPC shall
                         pay Immunex such costs within thirty (30) days after
                         Immunex provides Greenwich/AHPC an invoice detailing

                                       6
<PAGE>

                                                                  August 9, 2000

                         such costs during a calendar quarter in which such
                         technical assistance was rendered. Greenwich/AHPC's
                         reimbursement costs under this Paragraph 6 shall be
                         included in the Purchase Price and paid by Immunex in
                         accordance with the provisions of Paragraph 3 above.

7.   STAFFING:           During the period of Greenwich/AHPC's ownership of the
                         Greenwich Shares, the parties shall consult in good
                         faith on all major hires at or relating to the
                         Facility, defined as manager and above.

                         With the exception of existing Immunex employees,
                         employees employed at the Facility are currently
                         Greenwich/AHPC employees. At the Closing, a still
                         undetermined number of such Greenwich/AHPC employees
                         will be offered employment by Immunex. Specific
                         provisions relating to severance liabilities will be
                         included in the Definitive Agreement.

                         The parties shall agree on a maximum number and the job
                         titles of personnel at the Facility before and after
                         the Closing as candidates for transfer to BIOS to
                         ensure continuity of operations at the Facility and the
                         successful construction, start-up and operation of
                         BIOS. Such number shall not exceed a reasonably small
                         percentage of such personnel, currently estimated to be
                         in the ten percent (10%) to fifteen percent (15%) range
                         of the total workforce at the Facility, which number
                         shall be agreed upon in the Definitive Agreement.

                         Greenwich/AHPC shall, except in extenuating
                         circumstances with Immunex's approval, such approval
                         not to be unreasonably withheld, provide at least nine
                         (9) months' prior notice to Immunex of those personnel
                         at the Facility who will be transferred to an AHPC
                         Affiliate such as BIOS to allow sufficient time to
                         recruit and to train replacement personnel at the
                         Facility.

8.   FACILITY USE:       Immunex will have complete access to the entire
                         Facility during the time of Greenwich/AHPC's

                                       7
<PAGE>

                                                                  August 9, 2000

                         ownership of the Greenwich Shares, subject to standard
                         safety and security procedures, to facilitate Immunex's
                         participation in the Project. As between the parties,
                         Immunex has the final authority with respect to
                         decisions related to Phase B, including general
                         timelines relating thereto, and with respect to
                         conceptual design, basic engineering and detailed
                         engineering. Such general timelines for Phase B shall
                         be conducted in a manner which does not impact the most
                         aggressive accelerated schedule for the retrofit of
                         Suites A and B (i.e., Phase A-3) of the Facility.

                         Greenwich/AHPC shall lease land to Immunex for Phase B
                         at nominal rent (i.e., $1.00 total per year) on a
                         triple net basis.

9.   DUE DILIGENCE:      Prior to the Closing, each party will have complete
                         access to all records, documents and any other
                         materials made available or otherwise furnished to the
                         other party relating to the Facility.

10.  LEGAL EFFECT:       This MOU is binding. It is anticipated that the
                         Definitive Agreement will contain representations,
                         warranties and indemnities substantially identical to
                         those received by Greenwich/AHPC from GW/GWBI in the
                         Purchase Agreement, with such revisions and
                         modifications as are appropriate under these
                         circumstances.

11.  EXPENSES:           Except as set forth herein or as otherwise agreed among
                         the parties, each party shall be solely responsible for
                         all of its own costs and expenses, including the
                         expenses of legal counsel, accountants and other
                         advisors and representatives, incurred at any time in
                         connection with pursuing the Purchase Agreement, the
                         Definitive Agreement and the Project.

12.  BROKERS:            Each of Greenwich/AHPC and Immunex agree that no broker
                         or finder was or is involved with the Purchase
                         Agreement, the Definitive

                                       8
<PAGE>

                                                                  August 9, 2000

                         Agreement or the Project, and each of Greenwich/AHPC
                         and Immunex agree to hold each other harmless to the
                         extent that any commission or finder's fees or claims
                         may become due relating to the Purchase Agreement, the
                         Definitive Agreement or the Project.

13.  SPECIFIC            The parties agree that irreparable damage would
     PERFORMANCE:        occur in the event any provision of this MOU was not
                         performed in accordance with the terms hereof and that
                         the parties shall be entitled to specific performance
                         of the terms of this MOU, in addition to any other
                         remedy at law or equity.

14.  SEVERABILITY:       If any provision of this MOU is held to be invalid or
                         unenforceable by a court of competent jurisdiction, all
                         other provisions shall continue in full force and
                         effect.

15.  WAIVER:             Any term or condition of this MOU may be waived or
                         qualified at any time by the party entitled to the
                         benefit thereof by a written instrument that
                         specifically identifies this MOU and the term or
                         condition to be waived or qualified and is executed by
                         a duly authorized officer of such party. No delay or
                         failure on the part of any party in exercising any
                         rights hereunder, and no partial or single exercise
                         thereof, shall constitute a waiver of such rights or of
                         any other rights hereunder.

16.  SUCCESSORS AND      The terms and provisions of this MOU shall inure
     ASSIGNS:            to the benefit of, and be binding upon, each party
                         hereto and their respective successors and assigns. Any
                         reference to Immunex, Greenwich, AHPC or Greenwich/AHPC
                         hereunder shall be deemed to include the successors and
                         assigns thereof.

17.  HSR ACT:            If required, prior to the Closing the parties will
                         prepare and make appropriate filings under Title II of
                         the Hart-Scott-Rodino Antitrust Improvements Act of
                         1976, as amended, and the rules promulgated thereunder
                         (16 C.F.R 801.1 et seq.) ("the Act") as soon as
                         reasonably practicable. The parties agree to co-operate
                         in the antitrust clearance process and to furnish
                         promptly to the FTC and the Antitrust Division of the
                         Department

                                       9
<PAGE>

                                                                  August 9, 2000

                         of Justice any additional information reasonably
                         requested by them in connection with such filings. The
                         purchase and sale of the Greenwich Shares shall not
                         occur until the earlier of (i) the waiting period
                         provided by the Act shall have terminated or shall have
                         expired without any action by any government agency or
                         challenge to the termination, or (ii) a determination
                         has been made by the parties that a filing under the
                         Act is not necessary.

If this MOU accurately sets forth our mutual understanding with respect to the
foregoing, please sign this MOU in the spaces provided below and return an
original to our attention.

                                        Very truly yours,

                                        IMMUNEX CORPORATION

                                        By: /s/ Edward V. Fritzky
                                           ----------------------------

                                        Name:  Edward V. Fritzky
                                             --------------------------

                                        Title:  Chief Executive Officer
                                              -------------------------

Acknowledged and agreed to:

AMERICAN HOME PRODUCTS
CORPORATION

By: /s/ Kenneth J. Martin
    -------------------------

Name:  Kenneth J. Martin
       ----------------------

Title:  Senior Vice President
        ---------------------

Acknowledged and agreed to:

GREENWICH HOLDINGS INC.

By: /s/ Jeffrey S. Sherman
    -------------------------

Name:  Jeffrey S. Sherman
       ----------------------

Title:  Vice President
        ---------------------

                                      10

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00014-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00014-of-00352.parquet"}]]