EXHIBIT 31.1


                        CERTIFICATION OF CEO PURSUANT TO
              SECURITIES EXCHANGE ACT RULES 13A-14(A) AND 15D-14(A)
                             AS ADOPTED PURSUANT TO
                  SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002

I, Terrance J. Bieker, certify that:

         1. I have  reviewed  this  annual  report  on Form  10-K  of  BioSource
International, Inc.;

         2. Based on my  knowledge,  this  report  does not  contain  any untrue
statement of a material fact or omit to state a material fact  necessary to make
the statements made, in light of the  circumstances  under which such statements
were made, not misleading with respect to the period covered by this report;

         3. Based on my knowledge, the financial statements, and other financial
information included in this report, fairly present in all material respects the
financial  condition,  results of operations and cash flows of the registrant as
of, and for, the periods presented in this report;

         4. The registrant's  other certifying officer and I are responsible for
establishing and maintaining  disclosure  controls and procedures (as defined in
Exchange Act Rules 13a-15(e) and 15d-15(e)) for the registrant and have:

                  a) Designed such disclosure controls and procedures, or caused
such disclosure controls and procedures to be designed under our supervision, to
ensure that  material  information  relating to the  registrant,  including  its
consolidated subsidiaries,  is made known to us by others within those entities,
particularly during the period in which this report is being prepared;

                  b) Evaluated the effectiveness of the registrant's  disclosure
controls and procedures and presented in this report our  conclusions  about the
effectiveness  of the disclosure  controls and procedures,  as of the end of the
period covered by this report based on such evaluation; and

                  c)  Disclosed  in this  report any change in the  registrant's
internal control over financial  reporting that occurred during the registrant's
most recent fiscal quarter (the  registrant's  fourth fiscal quarter in the case
of an annual report) that has materially  affected,  or is reasonably  likely to
materially affect, the registrant's  internal control over financial  reporting;
and

         5. The  registrant's  other  certifying  officer and I have  disclosed,
based  on  our  most  recent  evaluation  of  internal  control  over  financial
reporting,  to  the  registrant's  auditors  and  the  audit  committee  of  the
registrant's   board  of  directors  (or  persons   performing   the  equivalent
functions):

                  a) All significant deficiencies and material weaknesses in the
design or  operation  of internal  control over  financial  reporting  which are
reasonably  likely to  adversely  affect  the  registrant's  ability  to record,
process, summarize and report financial information; and

                  b)  Any  fraud,   whether  or  not  material,   that  involves
management or other  employees who have a significant  role in the  registrant's
internal control over financial reporting.

Date:    March 29, 2004
                                                 /S/ TERRANCE J. BIEKER
                                                 -------------------------------
                                                 Terrance J. Bieker
                                                 Chief Executive Officer