Document:

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                                                                    EXHIBIT 10.3

                             Secured Promissory Note
                             -----------------------

$279,417,982                                                    January 1, 2002

1.   FOR VALUE RECEIVED, Immunex Corporation, a Washington corporation
     ("Borrower"), hereby unconditionally promises to pay to American Home
       --------
     Products Corporation, a Delaware corporation ("AHP") and AHP Subsidiary
     Holding Corporation ("Holdings, and together with AHP, the "Payees"), the
                                                                 ------
     principal sum of Two Hundred Seventy Nine Million Four Hundred Seventeen
     Thousand and Nine Hundred Eighty-two Dollars ($279,417,982), in cash in
     immediately available funds, on the dates hereinafter specified, and to pay
     interest on the unpaid principal amount thereof in like money and funds,
     for the period commencing on the date hereof until the principal amount of
     this Note shall be paid in full, at the rates per annum and on the dates
     provided herein.

2.   As used herein, the following terms shall have the following meanings:

     "Bankruptcy Code" shall mean the United States Bankruptcy Code of 1978, as
      ---------------
      amended (as now or hereafter in effect).

     "Business Day" shall mean any day on which commercial banks are not
      ------------
     authorized or required to close in the City of New York.

     "Default" or "Event of Default" shall have the meanings given such terms in
      -------      ----------------
     Paragraph 6 hereof.

     "Dollars" or "$" shall mean lawful money of the United States of America.
      -------      -

     "Indebtedness" shall mean, as to any Person: (i) indebtedness created,
      ------------
     issued or incurred by such Person for borrowed money (whether by loan or
     the issuance and sale of debt securities); (ii) non-contingent obligations
     of such Person to pay the deferred purchase or acquisition price of
     property or services, other than trade accounts payable arising, and
     accrued expenses incurred, in the ordinary course of business so long as
     such trade accounts payable are payable within 90 days of the date the
     respective goods are delivered or respective services rendered; (iii)
     indebtedness of others secured by a security interest, mortgage, lien,
     encumbrance, collateral assignment or right of any third party in or of the
     property of such Person, whether or not the respective indebtedness so
     secured has been assumed by such Person; (iv) obligations of such Person in
     respect of letters of credit or similar instruments issued or accepted by
     banks and other financial institutions for the account of such Person; (v)
     capital lease obligations of such Person; and (vi) indebtedness of others
     guaranteed by such Person or for which such Person may be, or may become,
     liable.

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     "Insolvency Event" with respect to any Person shall mean that, and shall
      ----------------
     have occurred if:

     (a)  the Person shall: (1) apply for or consent to the appointment of, or
          the taking of possession by, a receiver, custodian, trustee or
          liquidator of itself or of all or a substantial part of its property;
          (2) make a general assignment for the benefit of its creditors;
          (3) commence a voluntary case under the Bankruptcy Code; (4) file a
          petition seeking to take advantage of any other law relating to
          bankruptcy, insolvency, reorganization, winding-up, or composition or
          readjustment of debts; (5) fail to controvert in a timely manner and
          appropriate manner, or acquiesce in writing to, any petition filed
          against it in an involuntary case under the Bankruptcy Code; or (6)
          take any corporate action for the purpose of effecting any of the
          foregoing;

     (b)  a proceeding or case shall be commenced, without the application or
          consent of the Person, in any court of competent jurisdiction,
          seeking: (1) its liquidation, reorganization, dissolution or
          winding-up, or the composition or readjustment of its debts; (2) the
          appointment of a trustee, receiver, custodian, liquidator or the like
          of the Person of all or any substantial part of its assets; or (3)
          similar relief in respect of the Person under any law relating to
          bankruptcy, insolvency, reorganization, winding-up, or composition or
          adjustment of debts, and such proceeding or case shall continue
          undismissed, or an order, judgment or decree approving or ordering any
          of the foregoing shall be entered and continue unstayed and in effect,
          for a period of sixty (60) or more days; or

     (c)  an order for relief against the Person shall be entered in an
          involuntary case under the Bankruptcy Code.

     "Interest Payment Date" shall mean January 11, 2002.
      ---------------------

     "Interest Rate" shall mean an annual rate of interest equal to the rate
      -------------
     publicly announced by JPMorgan Chase Bank in New York, New York as its
     30-day LIBOR rate in effect on the date hereof plus 25 basis points.
     Interest shall be calculated daily on the basis of a year of 360 days and
     the actual number of days for which interest is due.

     "Note" shall mean this Secured Promissory Note.
      ----

     "Obligations" shall mean any and all liabilities, obligations, covenants,
      -----------
     agreements and payments of or required to be made by Borrower under or
     pursuant to this Note and the Pledge Agreement.

     "Person" shall mean any individual, corporation, company, voluntary
      ------
     association, partnership, joint venture, trust, unincorporated organization
     or government (or any agency, instrumentality or political subdivision
     thereof).

                                      -2-

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     "Pledge Agreement" shall mean the Pledge Agreement dated as of the date
      ----------------
     hereof between Borrower and Payees.

     "Purchase Agreement" shall mean the Purchase Agreement, dated as of
      ------------------
     November 6, 2001, as amended (as amended, the "Purchase Agreement"), by
     and among the Payees and Borrower with respect to the issued and
     outstanding shares of capital stock of Greenwich Holdings Inc.

     "Subsidiary" shall mean, with respect to any Person, any other Person
      ----------
     of which at least a majority of the outstanding shares of stock or
     other ownership interests having by the terms thereof ordinary voting
     power to elect a majority of the board of directors or other similar
     management body of such other Person (irrespective of whether or not at
     the time stock or other ownership interests of any other class or
     classes of such other Person shall have or might have voting power by
     reason of the happening of any contingency) is at the time directly or
     indirectly owned or controlled by such first Person or one or more of
     the Subsidiaries of such first Person.

3.   (a)  The principal amount of this Note shall mature and be payable in full,
          on January 11, 2002.

     (b)  The principal amount of this Note may be prepaid by Borrower
          in whole or in part at any time and from time to time, without
          premium or penalty, provided that all accrued and unpaid
          interest on the principal amount of this Note being prepaid
          shall have been paid in full.

4.   (a)  Interest shall be payable on the principal amount of this Note
          outstanding from time to time at a rate equal to the Interest Rate
          during the period from and including the date hereof and payable, in
          arrears (calculated on the basis of a 360 day year), on January 11,
          2002.

     (b)  Any principal amount hereof which is not paid when due (whether at the
          stated maturity, by acceleration or otherwise), shall bear interest
          during the period from and including the date due to the date of
          payment in full at the rate per annum equal to the Interest Rate, both
          before and after judgment.

5.   Borrower hereby represents and warrants to Payees, as follows:

     (a)  Borrower is a corporation duly organized and validly existing under
          the laws of the State of Washington and has paid all excise taxes
          required by the Washington Department of Revenue.

     (b)  Borrower has full corporate power and authority to enter into the
          Note and the Pledge Agreement and to consummate the transactions
          contemplated hereby. The execution, delivery and performance by
          Borrower of this Note have been duly

                                      -3-

<PAGE>

          authorized by all requisite corporate action on the part of Borrower.
          This Note has been duly executed and delivered by Borrower, and
          constitutes a valid and binding obligation of Borrower, enforceable in
          accordance with its terms, except as such enforceability may be
          limited by bankruptcy, insolvency, reorganization or similar laws
          affecting creditors' rights generally or by general equitable
          principles.

     (c)  The execution, delivery and performance by Borrower of this Note do
          not and will not contravene or conflict with (i) the articles of
          incorporation or by-laws of Borrower; (ii) in any material respect,
          any mortgage, deed of trust, lease, note, contract, agreement, bond,
          indenture, license, permit or trust to which Borrower or any of its
          Subsidiaries is a party; or (iii) in any material respect, any
          judgment, order, writ, injunction or decree of any court, governmental
          body, governmental authority, or arbitrator, to which Borrower or any
          of its Subsidiaries is a party, that, in any case, would prevent or be
          violated by, or under which there would be a default as a result of,
          the execution, delivery and performance by Borrower of this Note and
          the consummation of the transactions contemplated hereby. No material
          consent, approval or authorization of or declaration or filing with
          any Person or governmental authority is required for the valid
          execution, delivery and performance by Borrower of this Note and the
          consummation of the transactions contemplated hereby.

6.   If one or more of the following events (herein called "Events of Default")
                                                            -----------------
     shall occur:

     (a)  Borrower shall default in the payment when due of any principal of or
          interest on this Note; or

     (b)  Borrower shall default in the payment when due of any other amount
          payable by it under this Note or of any other monetary Obligation, and
          such default in payment shall continue unremedied for a period of
          three (3) Business Days after notice thereof by Payees to Borrower; or

     (c)  Borrower shall default in the performance of any of its Obligations
          (other than Obligations to pay money), and such default shall continue
          unremedied for a period of ten (10) calendar days after notice thereof
          by Payees to Borrower; or

     (d)  an Insolvency Event with respect to Borrower or any of its
          Subsidiaries; or

     (e)  the occurrence of any "default" or "event of default" under any
          agreement, indenture or instrument evidencing or governing
          Indebtedness of Borrower in a principal amount outstanding of at
          least $25,000,000 in the aggregate for Borrower;

thereupon,

                                      -4-

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     (x)  in the case of an Event of Default (other than an Insolvency Event),
          Payees may, by notice to Borrower, declare the principal amount then
          outstanding of, and the accrued interest on, this Note and all other
          amounts payable by Borrower under this Note to be forthwith due and
          payable, whereupon such amounts shall be immediately due and payable
          without presentment, demand, protest or other formalities of any kind,
          all of which are hereby waived by Borrower; and

     (y)  in the case of the occurrence of any Insolvency Event, the principal
          amount then outstanding of, and the accrued interest on, this Note
          and all other amounts payable by Borrower under this Note shall be
          automatically immediately due and payable without presentment, demand,
          protest or other formalities of any kind, all of which are hereby
          waived by Borrower.

7.   THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS
     OF THE STATE OF NEW YORK (OTHER THAN THE CHOICE OF LAW PRINCIPLES
     THEREOF).

8.   Any action, suit or other proceeding initiated by Payees to enforce this
     Note may be brought in any Federal or state court in the State of New York,
     as Payees shall elect, having jurisdiction over the subject matter thereof.
     Borrower hereby submits itself to the jurisdiction of any such court and
     irrevocably appoints Borrower's Chief Financial Officer at 51 University
     Street, Seattle, Washington 98101 as agent for service of process, and
     agrees that service of process on Borrower in any such action, suit or
     proceeding may be effected by certified mail/return receipt requested
     addressed to Borrower's Chief Financial Officer at the foregoing address.

9.   All notices, demands, requests or other communications that are required or
     permitted hereunder shall be in writing and sufficient if delivered
     personally, sent by telecopier, or sent by Federal Express or by registered
     or certified mail, postage prepaid, to the address or telecopier number, as
     the case may be, as follows:

     If to Payees:

     American Home Products Corporation
     Five Giralda Farms
     Madison, New Jersey 07940
     Attn: Executive Vice President and General Counsel
     Telecopier No.: (973) 660-7156
     Telephone No.:  (973) 660-5000

                                      -5-

<PAGE>

     If to Borrower:

     Immunex Corporation
     51 University Avenue
     Seattle, Washington 98101
     Attn: General Counsel
     Telecopier No: (206) 292-9271
     Telephone No.: (206) 587-0430

     All such notices or other communications shall be deemed to have been given
     on the date transmitted by telecopier or personally delivered, or, in the
     case of notice or communication sent by Federal Express, one (1) day after
     the date deposited with Federal Express, or, in the case of notice or
     communication sent by registered or certified mail, seven (7) days after
     the date deposited in the mails, in each case given or addressed as
     aforesaid. Any party may by notice to each of the other parties change the
     address to which notice or other communications to it are to be delivered
     or mailed.

10.  Any waiver of any term or condition of this Note, or any amendment or
     supplementation of this Agreement, shall be effective only if in writing. A
     waiver of any breach or failure to enforce any of the terms or conditions
     of this Agreement shall not in any way affect, limit or waive a party's
     rights hereunder at any time to enforce strict compliance thereafter with
     every term or condition of this Agreement.

11.  Borrower shall pay on demand all costs and expenses (including without
     limitation reasonable legal fees) incurred by Payees in connection with the
     enforcement of this Note, and, upon such demand by Payees, such costs and
     expenses shall become Obligations of Borrower under this Note.

12.  The Obligations of Borrower hereunder (whether for the payment of principal
     and interest or otherwise, and whether upon maturity or acceleration) shall
     be due, payable and performable by Borrower without the necessity of
     presentment, demand, notice, protest or other formalities of any kind, all
     of which are hereby waived by Borrower; provided that the foregoing shall
     in no way be deemed to limit the obligation of Payees to provide notice of
     default under Paragraph 6(a) hereof. Borrower agrees to pay all amounts
     owing under this Note without set-off, counterclaim, or defense of any
     nature whatsoever and, in any litigation arising out of or relating to this
     Note or to the payment of any portion thereof in which the holder of this
     Note and Borrower shall be adverse parties, Borrower hereby waives the
     right to interpose any set-off, counterclaim, or defense whatsoever (other
     than the defense of actual payout).

13.  This Note and the rights and obligations hereunder shall not be assignable
     or transferable by the Borrower or the Payees, provided, however, that
     either Payee may assign its rights and obligations hereunder to an
     Affiliate of such Payee (as the term "Affiliate" is defined

                                      -6-

<PAGE>

     in the Purchase Agreement). Any attempted assignment in violation of this
     Section 13 shall be void.

                                 *     *    *

                                      -7-

<PAGE>

     In witness whereof, the undersigned has executed and delivered this Note,
in the State of Washington, as of the 1st day of January, 2002.

Attest:                               IMMUNEX CORPORATION

/s/ Angela M. Trout                   By:  /s/ David A. Mann
-----------------------------------        ------------------------------------
Angela M. Trout                            David A. Mann
Residency at Seattle, WA                   Chief Financial Officer and
                                           Executive Vice President

[Notary Seal]

                                      -8-<PAGE>

                                                                    EXHIBIT 10.4

                                PLEDGE AGREEMENT

          PLEDGE AGREEMENT dated as of January 1, 2002 by and among American
Home Products Corporation, a Delaware corporation ("AHP"), AHP Subsidiary
Holding Corporation ("Holdings" and together with AHP, the "Pledgees") and
                                                            --------
Immunex Corporation, a Washington corporation ("Pledgor").
                                                -------

          Pledgor has, on this date, executed and delivered to Pledgees or their
assigns a promissory note dated the date hereof in the principal amount of Two
Hundred Seventy Nine Million Four Hundred Seventeen Thousand and Nine Hundred
Eighty-two Dollars ($279,417,982) (the "Note").
                                        ----

          The Pledgor is the sole beneficial owner of one thousand (1,000)
common shares, no par value per share, of the outstanding capital stock of
Greenwich Holdings Inc., a Delaware corporation (the "Company").
                                                      -------

          To induce the Pledgees to extend the credit to the Pledgor represented
by the Note and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the Pledgor has agreed to
mortgage, assign, pledge and grant a security interest in the Collateral (as
hereinafter defined) as security for the Secured Obligations (as hereinafter
defined). Accordingly, the parties hereto agree as follows:

          Section 1. Definitions.  Terms defined in the Note and not otherwise
                     -----------
defined herein are used herein as defined therein.  In addition, as used herein:

          "Collateral" shall have the meaning ascribed thereto in Section 3
           ----------                                             -------
     hereof.

          "Issuer" shall mean the Company.
           ------

          "Lien" shall mean, with respect to any asset, any mortgage, lien,
           ----
     pledge, charge, security interest or encumbrance of any kind in respect of
     such asset.

          "Pledged Shares" shall have the meaning ascribed thereto in Section
           --------------                                             -------
     3(a) hereof.

          "Purchase Agreement" shall mean the Purchase Agreement, dated as of
           ------------------
     November 6. 2001, as amended, by and among the Pledgees and Pledgor.

          "Secured Obligations" shall mean (a) all obligations from time to time
           -------------------
     owing by the Pledgor to the Pledgees under the Note and any increases,
     renewals or extensions thereof and (b) all amounts from time to time owing
     to the Pledgees by the Pledgor hereunder.

          "Subsidiary" of any Person shall mean (a) any corporation of which at
           ----------
     least a majority of the outstanding shares of stock having by the terms
     thereof ordinary voting power to elect a majority of the board of directors
     of such corporation (irrespective of

<PAGE>

     whether or not at the time stock of any other class or classes of such
     corporation shall have or might have voting power by reason of the
     happening of any contingency) is at the time directly or indirectly owned
     or controlled by such Person and/or one or more of the Subsidiaries of such
     Person, or (b) any partnership of which at least a majority of the
     partnership or other ownership interests having by the terms thereof
     ordinary voting power to direct or cause the direction of management or
     policies of such partnership is at the time directly or indirectly owned by
     such Person and/or with respect to which such Person has the power,
     directly or indirectly, to direct or cause the direction of certain or all
     of the management and policies thereof.

          "Uniform Commercial Code" shall mean the Uniform Commercial Code as in
           -----------------------
     effect from time to time in the State of New York.

          Section 2. Representations and Warranties.  The Pledgor represents and
                     ------------------------------
 warrants to the Pledgees that, based solely upon the representations and
 warranties set forth in Section 4.2 of the Purchase Agreement:

          2.1. Pledged Shares.
               --------------

          (a) The Pledgor is the sole beneficial owner of the Pledged Shares. No
     Lien exists or will exist upon the Collateral at any time (and no right or
     option to acquire the same exists in favor of any other Person), except for
     the Lien in favor of the Pledgees created hereby. The Lien created hereby
     on the Collateral constitutes a first priority perfected pledge and
     security interest;

          (b) The Pledged Shares evidenced by the certificates identified on
     Annex 1 hereto are, and all other Pledged Shares will be, duly authorized,
     -----
     validly existing, fully paid and non-assessable and none of such Pledged
     Shares is or will be subject to any contractual restriction, or any
     restriction under the charter or by-laws of the Issuer thereof, upon the
     transfer of such Pledged Shares (except for any such restriction contained
     herein); and

          (c) The Pledged Shares evidenced by the certificates identified on
     Annex 1 hereto constitute all of the issued and outstanding shares of
     -----
     capital stock of any class of the Issuer thereof identified on said Annex 1
                                                                         -----
     beneficially owned by the Pledgor on the date hereof (whether or not
     registered in the name of the Pledgor), and said Annex 1 correctly
                                                      -----
     identifies, as of the date hereof, the Issuer of such Pledged Shares, the
     class and par value of such Pledged Shares and the number of shares
     evidenced by such certificates; and the Pledgor is the registered owner of
     all such shares.

                                      -2-

<PAGE>

          Section 3. The Pledge. As collateral security for the prompt payment
                     ----------
in full when due (whether at stated maturity, by acceleration or otherwise), of
the Secured Obligations, the Pledgor hereby mortgages, assigns, pledges and
grants to the Pledgees a security interest in, to and under all of the Pledgor's
right, title and interest in the following property, whether now owned by the
Pledgor or hereafter acquired and whether now existing or hereafter coming into
existence (all being collectively referred to herein as "Collateral"):
                                                         ----------

          (a)  the shares of capital stock of the Issuer identified on Annex 1
                                                                       -----
     hereto evidenced by the certificates now or hereafter owned by the Pledgor,
     together with, in each case, the certificates representing the same
     (collectively, the "Pledged Shares");
                         --------------

          (b)  all shares, securities, money or property representing a dividend
     on any of the Pledged Shares, or representing a distribution or return of
     capital upon or in respect of the Pledged Shares, or resulting from a
     split-up, revision, reclassification or other like change of the Pledged
     Shares or otherwise received in exchange therefor, and any subscription
     warrants, rights or options issued to the holders of, or otherwise in
     respect of, the Pledged Shares;

          (c)  without affecting the obligations of the Pledgor or the Company
     under any provision prohibiting such action hereunder or under the Note, in
     the event of any consolidation, merger or amalgamation in which any Issuer
     of the Pledged Shares is not the surviving corporation, all shares of each
     class in the capital of the successor corporation, formed by or resulting
     from such consolidation, merger or amalgamation; and

          (d)  all proceeds of and to any of the property of the Pledgor
     described in clauses (a) through (c) above in this Section 3 and, to the
                                                        -------
     extent related to any property described in said clauses or such proceeds,
     all books, correspondence, credit files, records, invoices and other
     papers.

          Section 4. Further Assurance; Remedies. In furtherance of the grant of
                     ---------------------------
the pledge and security interest pursuant to Section 3 hereof, the Pledgor
                                             -------
hereby agrees with the Pledgees as follows:

          4.1. Delivery and Other Perfection.  The Pledgor shall:
               -----------------------------

          (a)  Subject to the rights of the Pledgor under Section 4.3 hereof, if
                                                          -------
     any of the above-described shares, securities, money or property required
     to be pledged by the Pledgor under clauses (a), (b) and (c) of Section 3
                                                                    -------
     hereof are received by the Pledgor, forthwith either (i) transfer and
     deliver to the Pledgees such shares, securities, money or property so
     received by the Pledgor (together with the certificates for any such shares
     and securities duly endorsed in blank or accompanied by undated powers duly
     executed in blank) all of which thereafter shall be held by the Pledgees,
     pursuant to the terms of this Agreement, as part of the Collateral or (ii)
     take such other action as the Pledgees shall deem reasonably necessary or
     appropriate to duly record the Lien created hereunder in such shares,
     securities, money or property referred to in said clauses (a), (b) and (c);

                                      -3-

<PAGE>

          (b)  give, execute, deliver, file and/or record any financing
     statement, notice, instrument, document, agreement or other papers that may
     be reasonably necessary or desirable (in the judgment of the Pledgees), to
     create, preserve, perfect or validate the security interest granted
     pursuant hereto or to enable the Pledgees to exercise and enforce its
     rights hereunder with respect to such pledge and security interest,
     including, without limitation, upon the occurrence and continuation of a
     Default, causing any or all of the Collateral to be transferred of record
     into the name of the Pledgees or their nominees (and the Pledgees agree
     that if any Collateral is transferred into its name or the name of its
     nominee, the Pledgees will thereafter promptly give to the Pledgor copies
     of any notices and communications received by it with respect to the
     Collateral); and

          (c)  permit representatives of the Pledgees to have access in the same
     manner as is provided for in Section 7.2 of the Purchase Agreement.

          4.2. Preservation of Rights. The Pledgees shall not be required to
               ----------------------
take steps necessary to preserve any rights against prior parties to any of the
Collateral.

          4.3. Rights of Ownership.
               -------------------

          (a)  So long as no Event of Default shall have occurred and be
     continuing, the Pledgor shall have the right to exercise all voting,
     consensual and other powers of ownership pertaining to the Collateral for
     all purposes not inconsistent with the terms of this Agreement or the Note,
     provided that the Pledgor agrees that it will not vote the Collateral in
     any manner that is inconsistent with the terms of this Agreement or the
     Note; and the Pledgees shall execute and deliver to the Pledgor or cause to
     be executed and delivered to the Pledgor all such proxies, powers of
     attorney, dividends and other orders, and all such instruments, without
     recourse, as the Pledgor may reasonably request for the purpose of enabling
     the Pledgor to exercise the rights and powers that it is entitled to
     exercise pursuant to this Section 4.3(a);
                               -------

          (b)  Unless and until an Event of Default has occurred and is
     continuing, the Pledgor shall be entitled to receive and retain for its own
     use any dividends and interest on the Collateral paid in cash; and

          (c)  If any Event of Default shall have occurred, then so long as such
     Event of Default shall continue, all dividends and other distributions on
     the Collateral shall be paid directly to the Pledgees and retained by them
     as part of the Collateral, subject to the terms of this Agreement, and, if
     the Pledgees shall so request in writing, the Pledgor agrees to execute and
     deliver to the Pledgees appropriate additional dividend, distribution and
     other orders and documents to that end, provided that if such Event of
     Default is cured, any such dividend, interest or distribution theretofore
     paid to the Pledgees shall, upon request of the Pledgor, be returned by the
     Pledgees to the Pledgor.

          4.4. Events of Default, Etc. During the period during which an Event
               ----------------------
of Default shall have occurred and be continuing:

          (a)  the Pledgees shall have all of the rights and remedies with
     respect to the Collateral of a secured party under the Uniform Commercial
     Code (whether or not said

                                      -4-

<PAGE>

     Code is in effect in the jurisdiction where the rights and remedies are
     asserted), and such additional rights and remedies to which a secured party
     is entitled under the laws in effect in any jurisdiction where any rights
     and remedies hereunder may be asserted, including, without limitation, the
     right, to the maximum extent permitted by law, to exercise all voting,
     consensual and other powers of ownership pertaining to the Collateral as if
     the Pledgees were the sole and absolute owner thereof (and the Pledgor
     agrees to take all such action as may be appropriate to give effect to such
     right);

          (b) the Pledgees in their discretion may, in their name or in the name
     of the Pledgor or otherwise, demand, sue for, collect or receive any money
     or property at any time payable or receivable on account of or in exchange
     for any of the Collateral, and may discharge, cancel or grant any
     acquittance with respect to any of the Collateral (whether or not any
     amounts thereof shall have been paid or the Pledgees or the Pledgor shall
     have received any, or adequate, consideration therefor), but shall be under
     no obligation to do so;

          (c) the Pledgees may, if an Event of Default is not cured within five
     (5) days after notice to the Pledgor, upon 10 Business Days' prior written
     notice to the Pledgor of the time and place, with respect to the Collateral
     or any part thereof which shall then be or shall thereafter come into the
     possession, custody or control of the Pledgees or any of their agents,
     sell, lease, assign or otherwise dispose of all or any part of such
     Collateral, at such place or places as the Pledgees deem best, and for cash
     or on credit, or for future delivery (without thereby assuming any credit
     risk), at public or private sale, without demand of performance or notice
     of intention to effect any such disposition or of time or place thereof
     (except such notice as is required above or by applicable statute and
     cannot be waived), and the Pledgees or anyone else may be the purchaser,
     lessee, assignee or recipient of any or all of the Collateral so disposed
     of at any public sale (or, to the extent permitted by law, at any private
     sale), and thereafter hold the same absolutely, free from any claim or
     right of whatsoever kind, including any right or equity of redemption
     (statutory or otherwise), of the Pledgor, any such demand, notice or right
     and equity being hereby expressly waived and released. The Pledgees may,
     without notice or publication, adjourn any public or private sale or cause
     the same to be adjourned from time to time by announcement at the time and
     place fixed for the sale, and such sale may be made at any time or place to
     which the same may be so adjourned; and

          (d) The proceeds of each collection, sale or other disposition under
     this Section 4.4 shall be applied in accordance with Section 4.7 hereof.
          -------                                         -------

          The Pledgor recognizes that, by reason of certain prohibitions
contained in the Securities Act of 1933, as amended, and other applicable
securities laws, the Pledgees may be compelled, with respect to any sale of all
or any part of the Collateral, to limit purchasers to those who will agree,
among other things, to acquire the Collateral for their own account, for
investment and not with a view to the distribution or resale thereof. The
Pledgor acknowledges that any such private sales may be at prices and on terms
less favorable to the Pledgees than those obtainable through a public sale with
such restrictions, and, notwithstanding such circumstances, agrees that any such
private sale shall be deemed to have been made in a commercially reasonable
manner and that the Pledgees shall have no obligation to engage in

                                      -5-

<PAGE>

public sales and no obligation to delay the sale of any Collateral for the
period of time necessary to permit the issuer thereof to register it for public
sale.

          4.5. Deficiency. If the proceeds of sale, collection or other
               ----------
realization of or upon the Collateral pursuant to Section 4.4 hereof are
                                                  -------
insufficient to cover the costs and expenses of such realization and the payment
in full of the Secured Obligations, the Pledgor shall remain liable for any
deficiency.

          4.6. Private Sale. The Pledgees shall incur no liability as a result
               ------------
of the sale of the Collateral, or any part thereof, at any private sale pursuant
to Section 4.4 hereof conducted in a commercially reasonable manner. The Pledgor
   -------
hereby waives any claims against the Pledgees arising by reason of the fact that
the price at which the Collateral may have been sold at such a private sale was
less than the price which might have been obtained at a public sale or was less
than the aggregate amount of the Secured Obligations, even if the Pledgees
accept the first offer received and does not offer the Collateral to more than
one offeree.

          4.7. Application of Proceeds. Except as otherwise herein expressly
               -----------------------
provided, the proceeds of any collection, sale or other realization of all or
any part of the Collateral pursuant hereto, and any other cash at the time held
by the Pledgees under this Section 4, shall be applied by the Pledgees:
                           -------

          First, to the payment of the costs and expenses of such collection,
          -----
     sale or other realization, including reasonable out-of-pocket costs and
     expenses of the Pledgees and the reasonable fees and expenses of its agents
     and counsel, and all expenses, and advances made or incurred by the
     Pledgees in connection therewith;

          Next, to the payment in full of the Secured Obligations; and
          ----

          Finally, to the payment to the Pledgor, or its successors or assigns,
          -------
     or as a court of competent jurisdiction may direct, of any surplus then
     remaining.

As used in this Section 4, "proceeds" of Collateral shall mean cash, securities
                -------     --------
and other property realized in respect of, and distributions in kind of,
Collateral, including any thereof received under any reorganization, liquidation
or adjustment of debt of the Pledgor or any Issuer.

          4.8. Attorney-in-Fact. Without limiting any rights or powers granted
               ----------------
by this Agreement to the Pledgees while no Event of Default has occurred and is
continuing, upon the occurrence and during the continuance of any Event of
Default the Pledgees are hereby appointed the attorney-in-fact of the Pledgor
for the purpose of carrying out the provisions of this Section 4 and taking any
                                                       -------
action and executing any instruments which the Pledgees may deem necessary or
advisable to accomplish the purposes hereof, which appointment as
attorney-in-fact is irrevocable and coupled with an interest. Without limiting
the generality of the foregoing, so long as the Pledgees shall be entitled under
this Section 4 to make collections in respect of the Collateral, the Pledgees
     -------
shall have the right and power to receive, endorse and collect all checks made
payable to the order of the Pledgor representing any dividend interest payment
or other distribution in respect of the Collateral or any part thereof and to
give full discharge for the same.

                                      -6-

<PAGE>

          4.9.  Perfection. Prior to or concurrently with the execution and
                ----------
delivery of this Agreement, the Pledgor shall deliver to the Pledgees all
certificates representing the Pledged Shares on the date hereof, accompanied by
undated powers duly executed in blank.

          4.10. Termination. When all Secured Obligations shall have been paid
                -----------
in full this Agreement shall terminate, and the Pledgees shall forthwith cause
to be assigned, transferred and delivered, against receipt but without any
recourse, warranty or representation whatsoever, any remaining Collateral and
money received in respect thereof, to or on the order of the Pledgor.

          4.11. Expenses. The Pledgor agrees to pay to the Pledgees all
                --------
reasonable out-of-pocket expenses (including reasonable expenses for legal
services of every kind), of, or incident to, the enforcement of any of the
provisions of this Section 4, or performance by the Pledgees of any obligations
                   -------
of the Pledgor in respect of the Collateral which the Pledgor has failed or
refused to perform, or any actual or attempted sale, or any exchange,
enforcement, collection, compromise or settlement in respect of any of the
Collateral, and for the care of the Collateral and defending or asserting rights
and claims of the Pledgees in respect thereof, by litigation or otherwise and
all such expenses shall be Secured Obligations owing to the Pledgees secured
under Section 3 hereof.
      -------

          4.12. Further Assurances. The Pledgor agrees that, from time to time
                ------------------
upon the written request of the Pledgees, the Pledgor will execute and deliver
such further documents and do such other acts and things as the Pledgees may
reasonably request in order fully to effect the purposes of this Agreement.

          Section 5. Miscellaneous.
                     -------------

          5.1.  No Waiver. No failure on the part of the Pledgees or any other
                ---------
Secured Party or any of its agents to exercise, and no course of dealing with
respect to, and no delay in exercising, any right, power or remedy hereunder
shall operate as a waiver thereof; nor shall any single or partial exercise by
the Pledgees or any of its nominees or agents of any right, power or remedy
hereunder preclude any other or further exercise thereof or the exercise of any
other right, power or remedy. The remedies herein are cumulative and are not
exclusive of any remedies provided by law.

          5.2.  Governing Law. THIS AGREEMENT SHALL BE GOVERNED BY, AND
                -------------
CONSTRUED IN ACCORDANCE WITH, THE LAW OF THE STATE OF NEW YORK.

          5.3.  Notices. All notices, requests, consents and demands hereunder
                -------
shall be in writing and transmitted by telecopier, personally delivered or sent
by Federal Express to the intended recipient at its address or telecopy number
specified beneath its signature hereto or at such other telecopy number or
address as shall be designated by either party in a notice to the other party,
and shall be deemed to have been given when transmitted by telecopier, or
personally delivered or, in the case of a mailed notice or a notice sent by
Federal Express, seven Business Days after the date deposited in the mails,
postage prepaid, in each case given or addressed as aforesaid.

                                      -7-

<PAGE>

          5.4. Waivers, etc. The terms of this Agreement may be waived, altered
               ------------
or amended only by an instrument in writing duly executed by the Pledgor and the
Pledgees. Any such amendment or waiver shall be binding upon the Pledgees, each
holder of any of the Secured Obligations and the Pledgor.

          5.5. Successors and Assigns. This Agreement shall be binding upon and
               ----------------------
inure to the benefit of the respective successors and assigns of the Pledgor,
the Pledgees and each holder of any of the Secured Obligations, provided,
however, that the Pledgor shall not assign or transfer its rights hereunder
without the prior written consent of the Pledgees.

          5.6. Pledgees. The Pledgees may employ agents and attorneys-in-fact in
               --------
connection herewith and shall not be responsible for the negligence or
misconduct of any such agent or attorneys-in-fact selected by it in good faith,
provided, however, that this section 5.6 shall not apply to action taken by
agents or attorneys-in-fact that creates any Lien on the Collateral or results
in a sale of the Collateral.

          5.7. Severability. If any provision hereof is invalid and
               ------------
unenforceable in any jurisdiction, then, to the fullest extent permitted by law
(a) the other provisions hereof shall remain in full force and effect in such
jurisdiction and (b) the invalidity or unenforceability of any provision hereof
in any jurisdiction shall not affect the validity or enforceability of such
provision in any other jurisdiction.

                            [Signature page follows]

                                      -8-

<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Pledge
Agreement to be duly executed as of the day and year first above written.

                                IMMUNEX CORPORATION

                                By:         /s/ David A. Mann
                                    ----------------------------------------
                                            David A. Mann
                                            Executive Vice President and
                                            Chief Financial Officer

                                Address for Notices:

                                Immunex Corporation
                                51 University Avenue
                                Seattle Washington 98101

                                Telecopy No.: (206) 292-9271
                                Telephone No.: (206) 587-0430
                                Attention: General Counsel

                                 AMERICAN HOME PRODUCTS CORPORATION

                                 By:        /s/ Thomas M. Nee
                                    ----------------------------------------
                                            Thomas M. Nee
                                            Vice President

                                 AHP SUBSIDIARY HOLDING CORPORATION

                                 By:        /s/ TIMOTHY T. SLATER
                                    ----------------------------------------
                                            Timothy T. Slater
                                            Vice President

                                 Address for Notices:

                                 American Home Products Corporation
                                 Five Giralda Farms
                                 Madison, New Jersey 07940

                                 Telecopy No.: (973) 660-7156
                                 Telephone No.: (973) 660-5000
                                 Attention: General Counsel

                                      -9-

<PAGE>

                                     Annex 1
                                     -------

                                 Pledged Shares
                                 --------------

                   Certificate          Registered                Number of
 Issuer               Number               Owner                    Shares
 ------               ------               -----                    ------
 Greenwich              3           Immunex Corporation         1,000 shares of
 Holdings Inc.                                                  Common Stock, no
                                                                   par value

                                      -10-

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