<PAGE>   1
 
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                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
 
                            ------------------------
 
                                  FORM 10-K/A
                               (AMENDMENT NO. 1)
 
[X] ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECU RITIES EXCHANGE
    ACT OF 1934
 
                  FOR THE FISCAL YEAR ENDED DECEMBER 31, 1996
 
                                       OR
 
[  ] TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
     EXCHANGE ACT OF 1934
 
                        COMMISSION FILE NUMBER 000-21930
 
                         BIOSOURCE INTERNATIONAL, INC.
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)
 
<TABLE>
<S>                                           <C>
                   DELAWARE                                     77-0340829
       (STATE OR OTHER JURISDICTION OF                       (I.R.S. EMPLOYER
        INCORPORATION OR ORGANIZATION)                     IDENTIFICATION NO.)
</TABLE>
 
                  820 FLYNN ROAD, CAMARILLO, CALIFORNIA, 93012
                    (ADDRESS OF PRINCIPAL EXECUTIVE OFFICES)
 
         ISSUER'S TELEPHONE NUMBER, INCLUDING AREA CODE: (805)987-0086
 
      SECURITIES REGISTERED PURSUANT TO SECTION 12(B) OF THE EXCHANGE ACT:
                                      NONE
 
      SECURITIES REGISTERED PURSUANT TO SECTION 12(G) OF THE EXCHANGE ACT:
                         COMMON STOCK, $0.001 PAR VALUE
                                (TITLE OF CLASS)
 
     Indicate by check mark whether the registrant (1) filed all reports
required to be filed by Section 13 or 15(d) of the Exchange during the preceding
12 months (or for such shorter period that the registrant was required to file
such reports), and (2) has been subject to such filing requirements for the past
90 days.
Yes [X]  No [ ].
 
     Indicate by check mark if there is no disclosure of delinquent filers in
response to Item 405 of regulation S-K is not contained herein, and will not be
contained, to the best of registrant's knowledge, in definitive proxy or
information statements incorporated by reference in Part III of this Form 10-K
or any amendment to this Form 10-K.  [ ]
 
     At March 31, 1997 the aggregate market value of the outstanding shares of
voting stock held by non-affiliates of the registrant was $58,404,346.
 
     At March 31, 1997 the registrant had 8,353,277 shares of Common Stock, par
value $0.001 per share, issued and outstanding.
 
                      DOCUMENTS INCORPORATED BY REFERENCE
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                                        1
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                         BIOSOURCE INTERNATIONAL, INC.
 
EXPLANATION FOR FORM 10K/A AMENDMENT NO. 1
 
AN AMENDED FORM 10K, FOR THE FISCAL YEAR DECEMBER 31, 1996, IS HEREBY BEING
FILED TO INSERT CERTAIN INFORMATION INADVERTENTLY OMITTED FROM THE ORIGINAL
FILING.
 
                                         
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                                   SIGNATURES
 
     Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the Registrant has duly caused this Report to be signed on
its behalf by the undersigned, thereunto duly authorized.
 
Date: April 15, 1997                      By:      /s/ ANNA M. ANDERSON
                                            ------------------------------------
                                                      Anna M. Anderson
                                                  Chief Financial Officer
 
Date: April 15, 1997                      By:    /s/ JAMES H. CHAMBERLAIN
                                            ------------------------------------
                                                    James H. Chamberlain
                                                   Chairman of the Board,
                                               President and Chief Executive
                                                           Officer
 
     In accordance with the Exchange Act, this report has been signed below by
the following person on behalf of the registrant and in the capacities and on
the date indicated:
 
<TABLE>
<CAPTION>
                   SIGNATURE                                     TITLE                DATE
-----------------------------------------------        -----------------------------------------
<C>                                                    <S>                       <C>
 
          /s/ LEONARD M. HENDRICKSON                   Director                  April 15, 1997
-----------------------------------------------
            Leonard M. Hendrickson
 
           /s/ DAVID J. MOFFA, PH.D.                   Director                  April 15, 1997
-----------------------------------------------
             David J. Moffa, Ph.D.
 
           /s/ JOHN R. OVERTURF, JR.                   Director                  April 15, 1997
-----------------------------------------------
             John R. Overturf, Jr.
 
              /s/ ROBERT D. WEIST                      Director                  April 15, 1997
-----------------------------------------------
                Robert D. Weist
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                                   SIGNATURES



          In accordance with Section 13 or 15(d) of the Exchange Act, the
Registrant has caused this amendment to report to be signed on its behalf by
the undersigned, thereunto duly authorized, this 15th day of April, 1997.



                                    BIOSOURCE INTERNATIONAL, INC.


                                    By: /s/ James H. Chamberlain
                                       ------------------------------------
                                       James H. Chamberlain
                                       Chairman of the Board, President
                                       and Chief Executive Officer