**Exhibit 31.1**

 

**Certification of CEO Pursuant toSecurities Exchange Act Rules 13a-14(a) and 15d-14 (a)as Adopted Pursuant toSection 302 of the Sarbanes-Oxley Act of 2002**

 

 I,Terrance J. Bieker, certify that:

 

1.             I have reviewed this quarterly report onForm 10-K of BioSource International, Inc.;

 

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

 

3.             Based on my knowledge, the financialstatements, and other financial information included in this report fairlypresent in all material respects the financial condition, results of operationsand cash flows of the Registrant as of the dates, and for the periods,presented in this report;

 

4.             The Registrant’s other certifying officerand I are responsible for establishing and maintaining disclosure controls andprocedures (as defined in Exchange Act Rules 13a-14(e) and 15d-14 (e)) for theRegistrant and we have:

 

a)             Designed such disclosure controls andprocedures, or caused such disclosure controls and procedures to be designedunder our supervision, to ensure that material information relating to theRegistrant, including its consolidated subsidiaries, is made known to us byothers within those entities, particularly during the period in which thisreport is being prepared;

 

b)            Designed such internal control over financialreporting, or caused such internal control over financial reporting to bedesigned under our supervision, to provide reasonable assurance regarding thereliability of financial reporting and the preparation of financial statementsfor external purposes in accordance with generally accepted accountingprinciples;

 

c)             Evaluated the effectiveness of theRegistrant’s disclosure controls and procedures, and presented in this reportour 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

 

d)            Disclosed in thisreport any change in the Registrant’s internal control over financial reportingthat occurred during the Registrant’s most recent fiscal quarter (theRegistrant’s fourth fiscal quarter in the case of an annual report) that hasmaterially affected, or is reasonably likely to materially affect, theRegistrant’s internal control over financial reporting.

 

5.             The Registrant’s other certifying officerand I have disclosed, based on our most recent evaluation, of internal controlover financial reporting, to the Registrant’s auditors and the audit committeeof the Registrant’s board of directors (or persons performing the equivalentfunctions):

 

a)             All significant deficiencies and materialweaknesses in the design or operation of internal control over financialreporting which are reasonably likely to adversely affect the Registrant’sability to record, process, summarize and report financial information; and

 

b)            Any fraud, whether or not material, thatinvolves management or other employees who have a significant role in theRegistrant’s internal control over financial reporting.

 

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Date:March 18, 2005

 
| /s/  TERRANCE J. BIEKER               |   |
|---------------------------------------|--------|
| Terrance  J. Bieker                   |
| President and Chief Executive Officer |

 

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