EXHIBIT 10.1

                               AMENDMENT NO. 1 TO
                        WARRANT TO PURCHASE COMMON STOCK
                                       OF
                          BIOSOURCE INTERNATIONAL, INC.

         THIS  AMENDMENT  NO.  1  TO  WARRANT  TO  PURCHASE  COMMON  STOCK  (the
"AMENDMENT"),  is  entered  into on this 7th day of June  2004,  by and  between
BioSource  International,  Inc., a Delaware  corporation  (the  "COMPANY"),  and
Genstar Capital Partners II, L.P. (the "Holder").

         A. The Holder is the holder of that certain  Warrant to Purchase Common
Stock dated as of February  15,  2000,  issued to the Holder by the Company (the
"WARRANT").

         B. In connection with the Holder's desire to extend the Expiration Date
of the Warrant (as defined therein) from February 15, 2005 to February 15, 2007,
the parties have agreed to amend certain terms of the Warrant in accordance with
the terms hereof.

         NOW,  THEREFORE,  in  consideration  of the premises and agreements set
forth herein, and for other good and valuable consideration,  the mutual receipt
and  sufficiency of which are hereby  acknowledged,  and intending to be legally
bound hereby, the parties hereby agree as follows:

1. The first  paragraph of the  Warrant,  which  commences  with the terms "THIS
CERTIFIES  THAT," is hereby  amended  and  restated  in its  entirety to read as
follows:

         "THIS  CERTIFIES  THAT, for value received by BioSource  International,
         Inc., a Delaware corporation (the "COMPANY"),  GenStar Capital Partners
         II, LP, or its permitted  registered assigns  ("HOLDER"),  is entitled,
         subject to the terms and  conditions  of this  Warrant,  at any time or
         from  time to time  after  the  issuance  date  of  this  Warrant  (the
         "EFFECTIVE  DATE"),  and before 5:00 p.m.  Pacific  Time on the seventh
         (7th)  anniversary of the Effective Date (the  "EXPIRATION  DATE"),  to
         purchase  from the  Company  1,262,542  shares of  Common  Stock of the
         Company, at a price per share of $9.00 (the "PURCHASE PRICE"). Both the
         number of shares of Common  Stock  purchasable  upon  exercise  of this
         Warrant and the Purchase  Price are subject to adjustment and change as
         provided herein."

         2. Except as  expressly  amended  and  restated  herein,  all terms and
conditions of the Warrant are hereby ratified,  confirmed and approved and shall
remain in full force and effect.  In the event of any conflict or  inconsistency
between this Amendment and the Warrant, this Amendment shall govern.





         IN WITNESS WHEREOF,  the parties hereto have executed this Amendment as
of the date first set forth above.

Genstar Capital Partners II, L.P.          BioSource International, Inc.

By:    Genstar Capital LLC
Its:   General Partner


/s/ Jean-Pierre Conte                      /s/ Terrance Bieker
- ---------------------------------          -------------------------------------
By:                                        By:

Jean-Pierre Conte                          Terrance Bieker
- ---------------------------------          -------------------------------------
Printed Name                               Printed Name

Managing Director                          Chief Executive Officer and President
- ---------------------------------          -------------------------------------
Title                                      Title


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