<PAGE>

                                                                   Exhibit 10.57

                                 PROMISSORY NOTE
                                 ---------------

$500,000.00

1.      Promise to Pay.
        --------------

        For value received, I, Brian M. McNamee ("Staff Member"), a married man,
        and I, Gillian D. McNamee, wife of Staff Member, promise to pay to the
        order of Amgen Inc., a Delaware corporation ("Payee"), at its office at
        One Amgen Center Drive, Thousand Oaks, CA 91320-1789, the sum of Five
        Hundred Thousand Dollars and No Cents ($500,000.00) (the "Principal"),
        payable in full on the earlier of five (5) years from date of execution
        of this Note or thirty (30) days from the date on which Staff Member
        ceases to be an employee of Payee, whichever first occurs, together with
        interest on the Principal from the date of this Note until such date as
        the Note is paid in full. Interest on this Note shall be computed as set
        forth below. The interest rate for the period from the date of this Note
        through December 31, 2001 (the "initial rate") is 5.00% per annum on the
        unpaid Principal. After December 31, 2001 the interest rate on this Note
        shall change as set forth below.

2.      Adjustable Interest Rate.
        ------------------------

        The interest rate shall be adjusted annually on January 1 of each year
        (the "Change Date") so as to equal the average interest rate designated
        as the "Introduction Rates" on adjustable rate loans as publicly offered
        by the banks and savings and loans in California as published by the Los
        Angeles Times in its Sunday edition. The rate shall be set using the
        rates published in the Los Angeles Times on the Sunday immediately
        preceding the Change Date. In the event that the "Introduction Rates"
        list is not published in the Los Angeles Times for any reason, then, in
        such event, the Payee shall establish the interest rate based on a
        survey by it of the introductory interest rates on adjustable loans
        offered by no fewer than five banking institutions located in Southern
        California that the Payee, in its sole discretion, deems representative
        of banking institutions in the Ventura and Los Angeles County areas.
        Payee shall give Staff Member notice if the interest rate shall be
        determined using this alternative method. Notwithstanding the foregoing,
        the interest rate shall never be increased or decreased on any single
        Change Date by more than one percentage point from the interest rate for
        the preceding 12 months. At no time during the term of this Note shall
        the annual interest rate exceed 8.00% per annum.

        Payee shall deliver or mail to Staff Member a notice of any changes in
        the adjustable interest rate on this Note and the amount of the Staff
        Member's semi-monthly payroll deductions before the effective date of
        any change. The notice shall include information required by law to be
        given to Staff Member and also the title and telephone number of a
        person who shall answer any questions Staff Member may have regarding
        the notice.

3.      Salary Deduction.
        ----------------

        The interest on this Note shall be payable by semi-monthly deductions
        from Staff Member's salary. The amount of such deductions shall
        initially be One Thousand Forty-One Dollars and Sixty-Seven Cents
        ($1,041.67) per installment; provided, however, that the manner of
        payment of this Note shall not be limited to deductions from Staff
        Member's salary. The amount of such deductions shall be adjusted
        annually concurrently with any adjustment in the interest rate on this
        Note to ensure that interest to be incurred during the ensuing calendar
        year shall be paid in twenty-four (24) equal payments. The first such

<PAGE>

        installment shall be on 06/15/01; the second installment shall be on
        06/30/01; and each successive installment shall be on the fifteenth and
        last days of each successive month until the Principal is repaid. Payee
        shall give Staff Member at least seven (7) days advance notice of any
        adjustment in the amount of said payroll deductions. Staff Member
        acknowledges and agrees that by executing this Note, Staff Member agrees
        to the payroll deductions described in this Note.

4.      Option to Convert.
        -----------------

        At the end of the term of this Note, Staff Member shall have the option
        to seek to convert this loan to a loan amortized over an additional
        five-year period by executing a new Promissory Note at terms to be
        mutually agreed upon by Staff Member and Payee. In the event that Staff
        Member and Payee are unable to reach agreement on such terms, this Note
        shall become immediately due and payable.

5.      Prepayment.
        ----------

        Staff Member may prepay without penalty this Note in whole or in part at
        any time. Any and all payments or prepayments under this Note may be
        made by Staff Member to Payee at the following address (or such other
        address as it designates in writing to Staff Member):

                            AMGEN INC.
                            One Amgen Center Drive
                            Thousand Oaks, California 91320-1789

                            Attention:  Accounting Manager

6.      Attorneys' Fees.
        ---------------

        Staff Member agrees to pay all costs and expenses, including, without
        limitation, collection agency fees and expenses, reasonable attorneys'
        fees, costs of suit and costs of appeal, which Payee may incur in the
        exercise, preservation or enforcement of its right, powers and remedies
        hereunder, or under any documents or instruments securing this Note, or
        under law.

7.      Modification of Terms.
        ---------------------

        Payee may, with or without notice to Staff Member, cause additional
        parties to be added to this Note, or release any party to this Note, or
        revise, extend, or renew the Note, or extend the time for making any
        installment provided for by this Note, or accept any installment in
        advance, all without affecting the liability of Staff Member. Staff
        Member may not assign or transfer in any manner whatsoever this Note or
        any of Staff Member's obligations under this Note.

8.      Security Interest.
        -----------------

        The purpose of this loan is to purchase a personal residence. Staff
        Member shall secure this loan by executing and causing to be filed,
        immediately upon close of escrow, a trust deed on this residence,
        commonly known as 5921 Careybrook Drive, Agoura Hills, CA 91301 whose
        property description is as follows: Lot 79, of Tract No. 33411, in the
        City of

<PAGE>

        Agoura Hills, County of Los Angeles, State of California, as per map
        recorded in Book 1022 Page(s) 7 - 15 inclusive of Maps, in the office of
        the County Recorder of said County.

9.      Acceleration.
        ------------

        A) In the event Staff Member fails to pay when due any sums under this
        Note, then:

           (1) the entire unpaid balance of this Note shall, at the option of
           the Payee hereof, immediately become due and payable in full and
           unpaid Principal thereafter shall bear interest at the lesser of the
           maximum rate permitted by law or at the rate of 8.00% per annum; and

           (2) Staff Member authorizes Payee to deduct any sums due to Payee
           under this Note from any monies, including any wages due, otherwise
           owing to Staff Member.

        B) If Staff Member sells the residence which is purchased with the funds
        herein provided, this Note shall immediately become due and payable upon
        the sale of such residence.

10.     Waiver of Rights by Staff Member.
        --------------------------------

        Staff Member waives (1) presentment, demand, protest, notice of dishonor
        and/or protest and notice of non-payment; (2) the right, if any, to the
        benefit of, or to direct the application of, any security hypothecated
        to Payee until all indebtedness of Staff Member to Payee, however
        arising, has been paid; and (3) the right to require the Payee to
        proceed against any party to this Note, or to pursue any other remedy in
        Payee's power. Payee may proceed against Staff Member directly and
        independently of any other party to this Note, and the cessation of the
        liability of any other party for any reason other than full payment, or
        any revision, renewal, extension, forbearance, change of rate of
        interest, or acceptance, release or substitution of security, or any
        impairment or suspension of Payee's remedies or rights against any other
        party, shall not in any way affect the liability of Staff Member.

11.     Obligations of Persons Under this Note.
        --------------------------------------

        If more than one person signs this Note, each person is fully and
        personally obligated to keep all of the promises made in this Note,
        including the promise to pay the full amount owed. Any person who is a
        guarantor, surety, or endorser of this Note is also obligated to do
        these things. Any person who takes over these obligations, including the
        obligations of a guarantor, surety or endorser of this Note, is also
        obligated to keep all of the promises made in this Note. Payee may
        enforce its rights under this Note against each person individually or
        against all of the signatories to this Note. This means that any one of
        the signatories to this Note may be required to pay all of the amounts
        owed under this Note.

12.     Governing Law.
        -------------

        This Note and the obligations under this Note of Staff Member or any
        other signatory to this Note shall be governed by and interpreted and
        determined in accordance with the laws of the State of California as
        applied to contracts between California residents entered into and to be
        performed entirely within said State.

<PAGE>

IN WITNESS WHEREOF, the undersigned has/have executed and delivered this Note as
of the 30th day of May, 2001.

                                          /s/ Brian M. McNamee
                                          ______________________________________
                                          Brian M. McNamee


                                          /s/ Gillian D. McNamee
                                          ______________________________________
                                          Gillian D. McNamee