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                                                                   Exhibit 10.21

                          BioSource International, Inc.
                                 540 Flynn Road
                           Camarillo, California 93012

September 18, 2000

PRIVATE AND CONFIDENTIAL

Mr. Russell D. Hays
9 Stratford Way
Lincoln, MA 01773

Dear Russ:

Reference is made to that certain letter agreement dated August 2, 2000 (signed
by you on August 9, 2000) between and among BioSource International, Inc.
("BioSource"), you and Genstar Capital, LLC. For good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged, the
parties agree that the first full paragraph appearing on the top of page 3 of
the letter agreement is hereby amended to read as follows:

      Should there be a "change of control," all stock options that have been
      granted to you will vest immediately. If you are terminated for any reason
      other than for cause you will receive one year's salary and health care
      benefits over the regular pay period.

      A "change of control" means (i) any person or entity (or group of related
      persons or entities acting in concert) unaffiliated with Genstar Capital
      LLC ("Genstar") shall acquire shares of capital stock of the Company
      entitled to exercise 40% or more of the total voting power represented by
      all shares of capital stock of the Company then outstanding; or (ii) the
      Company shall enter into an agreement to sell or otherwise transfer all or
      substantially all of its assets or enter into an agreement to merge,
      consolidate or reorganize with any other corporation or entity, as the
      result of which less than 60% of the total voting power represented by the
      capital stock or other equity interests of the corporation or entity to
      which the Company's assets are sold or transferred or surviving such
      merger, consolidation or reorganization shall be held by the persons and
      entities who were holders of common stock of the Company on the date
      immediately prior to such transaction; or (iii) the Company shall issue to
      a party or parties unaffiliated with Genstar Capital LLC ("Genstar")
      otherwise than on a pro rata basis additional shares of capital stock
      representing (after giving effect to such issuance) more than 40% of the
      total voting power of the Company; or (iv) the persons who were the
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      directors of the Company as of September 18, 2000 shall cease to comprise
      a majority of the Board of Directors of the Company.

      "For cause" means delivery of written notice of termination by the Company
      "for cause," by reason of: (i) any act or omission knowingly undertaken or
      omitted by you with the intent of causing damage to the Company, its
      properties, assets or business or its stockholders, officers, directors or
      employees; (ii) any act by you involving a material personal profit to
      you, including, without limitation, any fraud, misappropriation or
      embezzlement, involving properties, assets or funds of the Company or any
      of its subsidiaries; (iii) your consistent failure to perform your normal
      duties or any obligation, as directed by the Board; (iv) your conviction
      of, or pleading nolo contendere to, (A) any crime or offense involving
      monies or other property of the Company; (B) any felony offense; or (C)
      any crime of moral turpitude; or (v) your chronic or habitual use of drugs
      or consumption of alcoholic beverages.

In all other respects, the letter agreement shall remain unchanged.

Sincerely,

BIOSOURCE INTERNATIONAL, INC.


/s/ Jean-Pierre L. Conte
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Jean-Pierre L. Conte
Director

Accepted this 18th day of September, 2000 Accepted this 18th day of September,
2000


/s/ Jean-Pierre L. Conte                /s/ Russell D. Hays
-----------------------------           ----------------------------
Jean-Pierre L. Conte                    Russell D. Hays
Managing Director
Genstar Capital, LLC