<PAGE>

                                                                   Exhibit 10.10

                       SEPARATION AND CONSULTING AGREEMENT

      THIS SEPARATION AND CONSULTING AGREEMENT ("Agreement") is made and entered
into as of September 18, 2000, by and between James H. Chamberlain
("Executive"), and BioSource International, Inc., a Delaware Corporation, and
its parent, successors, predecessors, affiliates and related entities (the
"Company").

                                    RECITALS

      A. Executive's employment with the Company will be terminated effective
with Executive's resignation on the date that Russell Hays commences his
employment as President and Chief Executive Officer of the Company (the
"Effective Date"), and from and after the Effective Date, Executive will
continue to serve as a consultant to the Company, as set forth herein.

      B. Executive and the Company want to settle fully and finally all
potential differences or differences between them, including all potential
differences or differences which arise out of or relate to Executive's
employment or resignation of employment with the Company.

                                    AGREEMENT

      NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein, Executive and the Company understand and agree as follows:

      1.    Executive's Resignation of All Positions With the Company.
            ---------------------------------------------------------

            (a) Executive hereby resigns any and all of his positions as an
officer of employee with the Company as of the Effective Date. These positions
include President, Chief Executive Officer and Board of Director of the Company.

            (b) From and after the Effective Date, Executive agrees to make
himself available from time to time to consult with the Company under the
direction of Russell Hays and other Company personnel. Mr. Chamberlain's
consulting duties shall include: (i) attending meetings and making introductions
on behalf of Mr. Hays and others to analysts and others in the financial
community; (ii) attending meetings and making introductions on behalf of Mr.
Hays and others to the Company's distributors, suppliers, vendors and other
similar parties; (iii) discussing with Mr. Hays and others the historic
operations of the Company; and (iv) generally facilitating during the term of
this Agreement an orderly transition to the new management team led by Mr. Hays.

      2.    Severance By the Company, Continuing Salary and Medical Benefits;
            ----------------------------------------------------------------
            Consulting Fees.
            ---------------

            (a) The Company agrees that provided that at least eight days have
elapsed between the Company's receipt of an original executed copy of this
Agreement and the Effective Date, and provided that Executive has not revoked
the release contained in Section 6(c) of this Agreement, it will: (i) continue
                         ------------
paying Executive's current base salary, as it is set forth in Section 3.1 of
                                                              -----------
Executive's employment agreement with the Company dated January 2, 1998 (the
"Employment
<PAGE>

Agreement"), less all applicable deductions/withholdings, for a period of
eighteen (18) months following the Effective Date; (ii) pay Executive a
pro-rated (January 1, 2000 through the Effective Date) "Bonus," as that term is
defined in Section 3.2 of the Employment Agreement, less all applicable
           -----------
deductions/withholdings in the amount of $75,000 on the Effective Date; (iii)
provide payment for Executive's continuation of his employment benefits, as
defined in Sections 3.4, 3.5 and 3.6 of the Employment Agreement, and for the
           -------------------------
extension of the applicable exercise period of under any stock option agreement
between Executive and the Company (but not in excess of the term of any such
option agreement), in each case for a period of eighteen (18) months following
the Effective Date; and (iv) pay to Executive the sum of $1,000.00 per month as
a consulting fee for a period of eighteen (18) months following the Effective
Date. After this eighteen-month period, Executive may continue the medical
coverage granted under Section 3.5 of the Employment Agreement through the
                       -----------
provisions of the Consolidated Omnibus Budget Reconciliation Act of 1985
(COBRA). Executive's base salary and bonus payments under this Agreement shall
be paid in the same manner and at the same times the Company pays base salaries
and bonuses to other executive officers of the Company.

            (b) The Company agrees that its receivable from Executive shall be
converted to a Term Loan pursuant to the terms and conditions of the Form of
Promissory Note attached to this Agreement as Exhibit A, which provide, among
                                              ---------
other things, that such note shall continue to bear interest at the rate of
5.9%, payable quarterly in arrears, and all principal shall become payable upon
the earlier to occur of (i) the fifth anniversary of the Effective Date, and
(ii) the date upon which Executive sells Common Stock of the Company having an
aggregate market value of in excess of $2,000,000.

            (c) The Company agrees to indemnify and hold Executive harmless from
and against any liability that Executive may suffer as a result of the lender,
the Small Business Administration (under its loan guarantee program) or the
California Statewide Development Corp. asserting any claim against Executive in
connection with Executive's guarantee of the Company's mortgages on the property
located at 820 Flynn Road, Camarillo, California.

            (d) Executive acknowledges that upon execution of this Agreement,
the terms described in this Section 2 shall constitute full and complete
                            ---------
satisfaction of any and all amounts properly due and owing to Executive as a
result of his employment with the Company and/or his resignation from that
employment and that in the absence of this Agreement, Executive would not be
entitled to some or all of such payments.

      3.    Non-Admission of Discrimination or Wrongdoing.
            ---------------------------------------------

            (a) This Agreement is entered into as a compromise of disputed
claims, and thus, the parties hereto expressly recognize that the making of this
Agreement shall not in any way be construed as an admission that the Company or
any individual has any liability to or acted wrongfully in any way with respect
to Executive or any other person. The Company specifically denies that it has
any liability to or that it has done any wrongful, harassing and/or
discriminatory acts against Executive or any other person on the part of itself,
or its subsidiaries, affiliates, predecessors, successors, officers, employees
or agents.

            (b) Executive understands and agrees that he has not suffered any
discrimination in terms, conditions or privileges of his employment based on
age, race, gender, religious creed,


                                       2
<PAGE>

color, national origin, ancestry, physical disability, mental disability,
medication condition, marital status, sexual orientation and/or sexual or racial
harassment. Executive understands and agrees that he has no claim for employment
discrimination under any legal or factual theory.

      4.    Company Trade Secrets, Confidential Information and Property.
            ------------------------------------------------------------

            (a) Executive agrees that he has had access to trade secrets and
other proprietary and confidential information pertaining to the Company.
Executive agrees that the provisions contained in Sections 5, 6, 7. 8 and 9 of
the Employment Agreement shall remain in full force and effect.

      5.    No Lawsuits or Claims.
            ---------------------

            Executive promises never to file a lawsuit, administrative
complaint, or charge of any kind with any court, governmental or administrative
agency or arbitrator against the Company or its officers, directors, agents or
employees, asserting any claims that are released in this Agreement. Executive
represents and agrees that, prior to signing this Agreement, he has not filed or
pursued any complaints, charges or lawsuits of any kind with any court,
governmental or administrative agency or arbitrator against the Company or its
officers, directors, agents or employees, asserting any claims that are released
in this Agreement.

      6.    Complete Release.
            ----------------

            (a) In consideration of the covenants and promises contained herein
and subject to the consideration set forth above in Section 2, Executive hereby
                                                    ---------
knowingly and voluntarily releases, absolves and discharges the Company and, as
applicable, its partners, attorneys, agents, officers, administrators,
directors, employees, affiliates, representatives, and/or assigns and
successors, past and present (collectively the "Releasees") from all rights,
claims, demands, obligations, damages, losses, causes of action and suits of all
kinds and descriptions, legal and equitable, known and unknown, that Executive
may have or ever have had against the Releasees from the beginning of time to
the date of execution of this Agreement, including, but not limited to, any such
rights, claims, demands, obligations, damages, losses, causes of action and
suits arising out of, during or relating to Executive's employment and/or his
resignation therefrom. The matters that are the subject of the releases referred
to in this paragraph shall be referred to collectively as the "Released
Matters." This includes, but is not limited to, claims for employment
discrimination, wrongful termination, constructive termination, violation of
public policy, breach of any express or implied contract, breach of any implied
covenant, fraud, intentional or negligent misrepresentation, emotional distress,
or any other claims relating to Executive's relationship with the Company.

            (b) Executive acknowledges and agrees that this Agreement represents
a compromise of known and unknown, asserted and unasserted, and actual and
potential claims, and that neither this Agreement nor any compliance herewith or
consideration given pursuant hereto, shall be construed as an admission by the
Company of any liability whatsoever, including, but not limited to, any
liability for any violation by the Company of any right of Executive or of any
person arising under any law, statute, duty, contract, covenant, or order, or
any liability for any act of age discrimination or other impermissible form of
harassment or discrimination by the Company against Executive or any other
person, as prohibited by any state or federal statute or common law, including,
but not limited to: (i) Title VII of the Civil Rights Act of 1964, 42 U.S.C.(S)
2000e; (ii) the
                                       3
<PAGE>

Americans With Disabilities Act, 42 U.S.C. (S)(S) 12101 et seq.; (iii) the Age
Discrimination in Employment Act, 29 U.S.C. (S)(S) 623 et seq.; (iv) the Family
and Medical Leave Act; 29 U.S.C. (S)(S) 2611 et seq.; (v) the California Fair
Employment and Housing Act; Cal. Gov't Codes (S) 12940 et seq.; (vi) the
California Workers' Compensation Act; Cal. Lab. Code(S)(S) 3600 et seq.; (vii)
the Fair Labor Standards Act; 29 U.S.C. (S)(S) 201 et seq.; (viii) the
Consolidated Omnibus Budget Reconciliation Act of 1985; 42 U.S.C. (S)(S) 201 et
seq.; (ix) Executive Order 11141 (age discrimination); (x) Section 503 of the
Rehabilitation Act of 1973; 29 U.S.C. (S) 701 et. seq.; (xi) the Employee
Retirement Income Security Act of 1974, 29 U.S.C.(S)(S) 1001 et seq.; and (xii)
the laws established by the California Division of Labor Standards Enforcement,
e.g., Cal. Lab. Codes (S)(S) 200-272, and that all such liability is expressly
disputed and denied.

            (c) Executive further understands and acknowledges that:

                  (1) this Agreement constitutes a voluntary waiver of any and
all rights and claims he has against the Releasees as of the date of the
execution of this Agreement, including rights or claims arising under the
Federal Age Discrimination in Employment Act of 1967 ("ADEA"), 29 U.S.C. (S)(S)
621 et seq., as amended by the Older Workers' Benefit Protection Act of 1990,
except for any allegation that a breach of this Act occurred following the
Effective Date;

                  (2) he has waived rights or claims pursuant to this Agreement
in exchange for consideration, the value of which exceeds the payment or
remuneration to which he was already entitled;

                  (3) he is hereby advised that he may consult with an attorney
of his choosing concerning this Agreement prior to executing it;

                  (4) he has been afforded a period of at least twenty-one (21)
days to consider the terms of this Agreement, and in the event he should decide
to execute this Agreement in fewer than twenty-one days, he has done so with the
express understanding that he has been given and declined the opportunity to
consider this Agreement for a full twenty-one days; and

                  (5) he may revoke this Section 6(c) of the Agreement at any
time during the seven (7) days following the date of execution of this
Agreement, and this Section 6(c) of the Agreement shall not become effective or
enforceable until such revocation period has expired. Executive further
understands and acknowledges that he may revoke only Section 6(c) of this
Agreement as it relates to any claim pursuant to the Federal Age Discrimination
in Employment Act, and that such revocation, if any, will not affect the
effectiveness or enforceability of any other of the Released Matters as they are
described in Section 6(c).

      7.    Unknown Claims.
            --------------

            Executive acknowledges that there is a risk that subsequent to the
execution of this Agreement, he will incur or suffer damage, loss or injury to
persons or property that is in some way caused by or connected with Executive's
employment or his resignation therefrom, but that is unknown or unanticipated at
the time of the execution of this Agreement. Executive does hereby specifically
assume such risk and agrees that this Agreement and the releases contained
herein shall and do apply to all unknown or unanticipated results of any and all
matters caused by or connected with Executive's employment or his resignation
therefrom, as well as those currently known or


                                       4
<PAGE>

anticipated. Accordingly, Executive acknowledges that he has read the provisions
of California Civil Code Section 1542, which provides as follows:

                  "A general release does not extend to claims which the
                  creditor does not know or suspect to exist in his favor at the
                  time of executing the release, which if known to him must have
                  materially affected his settlement with the debtor."

and that he expressly waives, relinquishes and forfeits all rights and benefits
accorded by the provisions of California Civil Code Section 1542, and
furthermore waives any rights that he might have to invoke said provisions now
or in the future with respect to the Released Matters.

      8.    Ownership of Claims.
            -------------------

            Executive represents and warrants that no portion of any of the
Released Matters and no portion of any recovery or settlement to which Executive
might be entitled has been assigned or transferred to any other person, firm,
entity or corporation not a party to this Agreement, in any manner, including by
way of subrogation or operation of law or otherwise. If any claim, action,
demand or suit should be made or instituted against the Releasees or any of them
because of any such purported assignment, subrogation or transfer, Executive
agrees to indemnify and hold harmless the Releasee(s) against such claim,
action, suit or demand, including necessary expenses of investigation,
attorneys' fees and costs.

      9.    Assumption of Risk; Investigation of Facts.
            ------------------------------------------

            (a) Executive hereby expressly assumes the risk of any mistake of
fact or that the true facts might be other than or different from the facts now
known or believed to exist, and it is Executive's express intention to forever
settle, adjust and compromise any and all disputes between and among him and the
Releasees, finally and forever, and without regard to who may or may not have
been correct in their respective understandings of the facts or the law relating
thereto.

            (b) In making and executing this Agreement, Executive represents and
warrants that he has made such investigation of the facts and the law pertaining
to the matters described in this Agreement as he deems necessary, and Executive
has not relied upon any statement or representation, oral or written, made by
any other party to this Agreement with regard to any of the facts involved in
any dispute or possible dispute between the parties hereto, or with regard to
any of his rights or asserted rights, or with regard to the advisability of
making and executing this Agreement.

      10.   No Representations.
            ------------------

            Executive represents and agrees that no promises, statements or
inducements have been made to him, which caused him to sign this Agreement other
than those expressly stated in this Agreement.


                                       5
<PAGE>

      11.   Non-Disparagement.
            -----------------

            Executive agrees that he will refrain from taking actions or making
statements, written or oral, which disparage or defame the goodwill or
reputation of the Company, and/or its directors, officers, executives and
employees or which could adversely affect the morale of other employees of the
Company and that Executive shall not demean or disparage the Company in any
communications or other dealings with any existing or potential employees,
customers, vendors and/or stockholders.

      12.   Successors.
            ----------

            This Agreement shall be binding upon Executive and upon his heirs,
administrators, representatives, executors, successors and assigns, and shall
inure to the benefit of the Company and to its administrators, representatives,
executors, successors and assigns.

      13.   Arbitration.
            -----------

            (a) Any claim or controversy arising out of or relating to this
Agreement or any breach thereof between Executive and the Company shall be
submitted to arbitration in Los Angeles County, California, before an
experienced employment arbitrator licensed to practice law in California and
selected in accordance with the Model Employment Arbitration Procedures of the
American Arbitration Association, as the exclusive remedy for such claim or
controversy. Either party desiring to arbitrate shall give written notice to the
other party within a reasonable period of time after the party becomes aware of
the need for arbitration. The decision of the arbitrator shall be final and
binding. Judgment on any award rendered by such arbitrator may be entered in any
court having jurisdiction over the subject matter of the controversy. The
prevailing party shall receive an award of costs and expenses related to the
arbitration, including attorneys' fees. The fees and costs of the arbitrator and
the cost of any record or transcript of the arbitration shall be borne by the
losing party.

            (b) Should Executive or the Company institute any legal action or
administrative proceeding with respect to any claim waived by this Agreement or
pursue any dispute or matter covered by this paragraph by any method other than
said arbitration, the responding party shall be entitled to recover from the
other party all damages, costs, expenses and attorneys' fees incurred as a
result of such action.

      14.   Tender of Severance Payments as a Condition to Challenge This
            -------------------------------------------------------------
            Agreement
            ---------

            Should Executive attempt to challenge the enforceability of this
Agreement, as a further limitation on any right to make such a challenge,
Executive shall initially submit to the Company the total proceeds provided to
him in connection with this Agreement plus interest at the standard statutory
rate, and invite the Company to retain such monies and agree with Executive to
cancel this Agreement. In the event the Company accepts this offer, the Company
shall retain such monies and this Agreement shall be canceled. In the event the
Company does not accept such offer, the Company shall so notify Executive and
shall place such monies into an interest-bearing escrow account pending
resolution of the dispute between Executive and the Company as to whether this
Agreement shall be set aside and/or otherwise rendered unenforceable.


                                       6
<PAGE>

      15.   Consultation With Counsel; Reasonable Time to Consider Agreement;
            ----------------------------------------------------------------
            Voluntary Participation in This Agreement.
            -----------------------------------------

            Executive represents and agrees that he has been advised of the
opportunity to review this Agreement with an attorney, that he has had the
opportunity to thoroughly discuss all aspects of his rights and this Agreement
with an attorney to the extent Executive elected to do so, that he has carefully
read and fully understands all of the provisions of this Agreement, that he has
been given a reasonable period of time to consider signing this Agreement, and
that he is voluntarily entering into this Agreement.

      16.   Severability And Governing Law.
            ------------------------------

            (a) Should any of the provisions in this Agreement be declared or be
determined to be illegal or invalid, all remaining parts, terms or provisions
shall be valid, and the illegal or invalid part, term or provision shall be
deemed not to be a part of this Agreement.

            (b) This Agreement is made and entered into in the State of
California and shall in all respects be interpreted, enforced and governed under
the laws of California, without regard to the conflicts of laws principles
thereof.

      17.   Entire Agreement.
            ----------------

            This Agreement constitutes the entire agreement between and among
the parties pertaining to the subject matter hereof and the final, complete and
exclusive expression of the terms and conditions of their agreement. Any and all
prior agreements, representations, negotiations and understandings made by the
parties, oral and written, express or implied, are hereby superseded and merged
herein.

      18.   Execution In Counterparts.
            -------------------------

            This Agreement may be executed in one or more counterparts, all of
which taken together shall constitute one agreement.

      19.   Attorneys' Fees.
            ---------------

            In any action or other proceeding to enforce rights hereunder, the
prevailing party shall receive an award of costs and expenses related to such
proceeding, including attorneys' fees.

      20.   Cooperativeness.
            ---------------

            All parties have cooperated in the drafting and preparation of this
Agreement, and it shall not be construed more favorably for or against any
party.


                                       7
<PAGE>

                                    EXECUTIVE:


                                    /s/ James H. Chamberlain
                                    --------------------------------
                                    JAMES H. CHAMBERLAIN


                                    BIOSOURCE INTERNATIONAL, INC.


                                    By: /s/ Russell D. Hays
                                       --------------------------------
                                          RUSSELL D. HAYS
                                    Its: Chief Executive Officer


                                       8
<PAGE>

                             Form of Promissory Note
                             -----------------------

$350,000
Los Angeles, California                                       September 18, 2000

      FOR VALUE RECEIVED, James H. Chamberlain ("Obligor"), promises to pay to
the order of BioSource International, Inc. ("Holder"), at Camarillo, California,
or such other place as may be designated from time to time by Obligee in
writing, the principal sum of Three Hundred and Fifty Thousand Dollars
($350,000), with interest on the unpaid principal from the date hereof at the
rate of five and nine tenths percent (5.9%) per annum. Interest shall be paid
quarterly in arrears on or before the last day of each calendar quarter. All
unpaid principal and interest shall be due and payable on the earlier to occur
of (i) the fifth anniversary of the Effective Date, and (ii) the date upon which
Executive sells Common Stock of the Company in one or more transactions having
an aggregate market value of in excess of $2,000,000.

      Obligor and all endorsers hereof hereby expressly waive presentment for
payment, notice of dishonor, protest and notice of protest, and agree that the
time for the payment or payments of any part of this Note may be extended
without releasing or otherwise affecting Obligor's liability on this Note.

      Obligor may prepay this Note at any time without penalty. If any amount
under this Note is not paid when due, Obligor promises to pay all costs of
collection and reasonable attorneys' fees incurred by the holder hereof on
account of such collection.

      This Note shall be governed by and construed and enforced in accordance
with the laws of the State of California, without regard to conflict of laws
principles.

      IN WITNESS WHEREOF, the undersigned has executed this Note on the day and
date first set forth above.

                                          OBLIGOR:


                                          _____________________________
                                          JAMES H. CHAMBERLAIN


                                       9