Document:

<PAGE>

                                                                   EXHIBIT 10.40

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

$250,000.00

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

     For value received, I, Fabrizio Bonanni ("Staff Member"), a married man,
     and I, Lorraine Bonanni, wife of Staff Member, promise to pay to the order
     of Amgen Inc., a Delaware corporation ("Payee"), at its office at Amgen
     Center, Thousand Oaks, CA 91320-1789, the sum of Two Hundred Fifty Thousand
     Dollars and No Cents ($250,000.00) (the "Principal"). Amgen will forgive
     twenty percent (20%) of the loan principle on each anniversary date of
     Staff Member's employment until the loan principle is paid in full. Each
     year the principle amount is reduced by twenty percent (20%), the interest
     only payments will be reduced accordingly. The loan amount that is forgiven
     each year will be reported as income to Staff Member and will be subject to
     federal, state, and other applicable taxes. If Staff Member ceases to be an
     employee of Payee, this Note shall become payable in full thirty (30) days
     from the date on which Staff Member ceases to be an employee of Payee,
     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, 1999 (the "initial rate") is 4.4% per
     annum on the unpaid Principal. After December 31, 1999 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 7.4% 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
     Four Hundred Fifty-Eight Dollars and Thirty-Three Cents ($458.33) 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
<PAGE>

     calendar year shall be paid in twenty-four (24) equal payments. The first
     such installment shall be on August 31, 1999; the second installment shall
     be on September, 15, 1999; 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.   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.
                    Amgen Center
                    Thousand Oaks, California 91320-1789

                    Attention:  Accounting Manager

5.   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.

6.   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.

7.   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 343
     South Beverly Glen, Los Angeles, CA  90024 whose property description is as
     follows:

          Lot 22 in Block 3 of Tract No. 7733, in the City of Los Angeles,
          County of Los Angeles, State of California, as per Map recorded in
          Book 130, Pages 28, 29 and 30 of Maps, in the Office of the County
          Recorder of said County.

8.   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

                                       2
<PAGE>

          interest at the lesser of the maximum rate permitted by law or at the
          rate of 7.4% 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.

9.   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.

10.  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.

11.  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.

                                       3
<PAGE>

IN WITNESS WHEREOF, the undersigned has/have executed and delivered this Note as
of the 7th day of August, 1999.

                                            /s/ Fabrizio Bonanni
                                            --------------------
                                            FABRIZIO BONANNI

                                            /s/ Lorraine Bonanni
                                            --------------------
                                            LORRAINE BONANNI

                                       4<PAGE>

                                                                   EXHIBIT 10.41

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

$250,000.00

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

    For value received, I, Fabrizio Bonanni ("Staff Member"), a married man, and
    I, Lorraine Bonanni, 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 Two Hundred Fifty
    Thousand Dollars and No Cents ($250,000.00) (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,
    1999 (the "initial rate") is 4.4% per annum on the unpaid Principal.  After
    December 31, 1999 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 7.4% 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
    Four Hundred Fifty Eight Dollars and Thirty Three Cents ($458.33) 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 installment shall be on November 30, 1999;
    the second installment shall be on December 15, 1999; 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
<PAGE>

    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 343
    South Beverly Glen Blvd., Los Angeles, CA  90024 whose property description
    is as follows:

        Lot 22 in Block 3 of Tract No. 7733, in the City of Los Angeles, County
        of Los Angeles, State of California, as per Map recorded in Book 130,
        Pages 28, 29 and 30 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:

                                       2
<PAGE>

      (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 7.4% 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.

                                       3
<PAGE>

IN WITNESS WHEREOF, the undersigned has/have executed and delivered this Note as
of the 29th day of October, 1999.

                                           /s/ Fabrizio Bonanni
                                           --------------------
                                           FABRIZIO BONANNI

                                           /s/ Lorraine Bonanni
                                           --------------------
                                           LORRAINE BONANNI

                                       4

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