Exhibit 2.9.1
                                                                   -------------

April 23, 2001

Mr. Paul A. Rosinack
President
Synbiotics Corporation
11011 Via Frontera
San Diego, CA 92127-1702

Dear Paul:

         This letter agreement sets forth the parties' mutual understandings and
agreement regarding the modification of certain of the terms of (i) the Secured
Promissory Note, dated as of April 18, 2000, made by Synbiotics Corporation
("Synbiotics") and payable to the order of Kirkegaard & Perry Laboratories, Inc.
("KPL") in the original principal amount of $1,000,000 (the "Note"), (ii) the
Security Agreement dated as of April 18, 2000, between Synbiotics and KPL (the
"Security Agreement"), and (iii) certain other related documents.

         The parties hereby agree:

                (i)  to enter into that certain Amendment to Secured Promissory
                     Note in the form attached hereto as Exhibit A (the
                                                         ---------
                     "Amendment");


                (ii) that Synbiotics shall take all any and all actions
                     necessary or appropriate to protect and preserve the
                     Collateral and KPL's rights therein and to defend the
                     Collateral against any and all claims and demands of all
                     persons at any time claiming the same or any interest
                     therein adverse to KPL;

         Except as modified by this letter agreement and the Amendment, each of
the Note, the Security Agreement, the Manufacturing Agreement and the Royalty
Agreement remain in full force and effect and are hereby ratified, confirmed and
approved.

         Please acknowledge your acceptance and agreement to the terms and
conditions of this letter agreement by signing in the space indicated below.

Sincerely,

                                        KIRKEGAARD & PERRY LABORATORIES INC.

                                        By:  /s/ Albert Perry
                                             ------------------
                                        Name: Albert Perry
                                        Title:  President

ACKNOWLEDGE AND AGREED:

SYNBIOTICS CORPORATION

By:  /s/ Paul A. Rosinack
     ----------------------
Name:  Paul A. Rosinack
Title:  President & CEO


                      AMENDMENT TO SECURED PROMISSORY NOTE

           THIS AMENDMENT TO SECURED PROMISSORY NOTE (this "Amendment") is
entered into effective as of April 17, 2001, by Synbiotics Corporation, a
California corporation (the "Borrower").

           WHEREAS, Borrower and Kirkegaard & Perry Laboratories, Inc.
("Holder") entered into an Asset Purchase Agreement dated as of April 18, 2000,
pursuant to which Borrower purchased certain assets of Holder and as payment of
a portion of the purchase price for such assets, issued to Holder a Secured
Promissory Note, dated April 18, 2000, payable to the order of Holder in the
original principal amount of $1,000,000 (the "Note"); and

           WHEREAS, the Borrower and Holder desire to amend the Note to, among
other things, modify the payment terms of the Note, such modification to be
effective as of April 18, 2001.

           NOW, THEREFORE, in consideration of the foregoing and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the Borrower and Purchaser hereby agree as follows:

           1. Defined Terms. For purposes of this Amendment, capitalized terms
not otherwise defined herein shall have the respective meanings set forth in the
Note (as amended hereby).

           2. Ratification of Obligations. Borrower hereby acknowledges,
ratifies and reaffirms all of its duties and obligations under the Note and the
Security Agreement, dated as of April 18, 2000, between Borrower and Holder (the
"Security Agreement"), including, but not limited to, the obligation to pay the
debt evidenced by the Note (the "Debt").

           3. Outstanding Balance. As of the date of this Amendment, the
outstanding principal balance of the Debt is $1,000,000.

           4. Payments. The payment schedule set forth in the first paragraph of
the Note is hereby amended in its entirety to read as follows:

           Said principal and interest due thereon shall be due and payable as
follows:

           Amount Due                 Payment Due Date
           ----------                 ----------------

           $200,000                   April 18, 2001

           $100,000                   Commencing May 18,
                                      2001 and each monthe
                                      thereafter until this
                                      Note is paid in full.

           5. Interest. A new second paragraph is added to Note which shall read
as follows:

                For the period commencing on April 18, 2001 until all sums due
           hereunder have been paid in full, interest will accrue on the unpaid
           principal balance outstanding from time to time at a fixed rate equal
           to 7.5% per annum and shall be payable in accordance with the payment
           schedule set forth above. Interest due hereunder shall be computed on
           the basis of a 360-day year and applied to the actual number of days
           elapsed.

           6. Accelerated Repayment. A new fifth paragraph is hereby added to
the Note which paragraph shall read as follows:

           "Upon the consummation of one or more financings by Borrower which
           results in aggregate gross proceeds to the Borrower of at least
           $3,000,000 after repayment of the Imperial Bank debt, the entire
           principal amount of this Note and all accrued but unpaid interest due
           thereon, shall become due and payable."


           "Upon the sale of all or substantially all of the equity interests in
           Borrower or all or substantially all of the assets of Borrower to a
           third party for consideration which is at least 35% in cash, the
           entire principal amount of this Note and all accrued but unpaid
           interest due thereon, shall become due and payable. In connection
           with such transaction, regardless of the cash percentage, Buyer shall
           assume all of the Borrower's obligations to the Security Agreement,
           Manufacturing Agreement, Royalty Agreement and all other obligations
           of Borrower to Holder."

           7. Security Interest. Borrower does hereby certify and confirm that
pursuant to the Security Agreement, Holder has a valid first priority lien on
the Collateral (as defined in the Security Agreement) for the full amount of the
Debt and interest now or hereafter owing thereon, and that there are no defenses
or offsets to said Security Agreement or the Note, and that all of the
provisions of the Note and the Security Agreement, except as modified herein or
by the letter agreement dated April 17, 2001, between Borrower and Holder (the
"Letter Agreement"), are unmodified and in full force and effect.

           8. Relationship of Parties. Borrower acknowledges and agrees that
Holder is not, has never been, and shall not be deemed a partner or joint
venturer of Borrower with respect to the Note, the Security Agreement and all
other agreements between Borrower and Holder, and that the relationship of
Holder to Borrower is, has always been, and shall continue to be strictly the
role of a secured creditor. Borrower hereby waives and relinquishes all claims,
demands, counterclaims and/or defenses alleging the existence of any
partnership, joint venture or other fiduciary relationship between Holder and
Borrower and Borrower holds Holder, its successors and assigns, harmless against
any and all losses, damages, penalties, fines forfeitures, legal fees and
related costs, judgments, and any other fees, costs and expenses that Holder may
sustain as a result of any such allegation by any person or entity whatsoever.

           9. Representations and Warranties. Borrower hereby represents and
warrants to and for the benefit of Holder that (i) Borrower is a California
corporation, duly organized and validly existing, and is in good standing in the
State of California and (ii) the execution, delivery and performance of this
Amendment has been duly authorized and properly executed and delivered and the
consent of any other person or entity to such execution, delivery and
performance required to render this Amendment a valid and binding agreement
enforceable in accordance with its terms has been obtained.

           10. Ratification. Except for the terms set forth herein and in the
Letter Agreement, all of the terms, covenants and conditions of the Note and the
Security Agreement shall remain in full force and effect and are hereby ratified
and confirmed. It is the express intention of the parties hereto that this
Amendment shall not constitute a novation of the Note or the Security Agreement.

           11. Binding Effect. All of the terms, covenant and conditions herein
contained are and shall be binding upon, and shall inure to the benefit of, the
parties hereto and their successors and assign.

           12. Severability. If any provision of the Note is invalid, illegal,
or unenforceable, then such provision shall be deemed severable from the rest of
the Note and the validity, legality, and enforceability of the remaining
provisions shall not in any way be affect or impaired thereby.

           13. Governing Law. This Amendment shall be construed in accordance
with the laws of the State of Maryland not including the choice of laws rules
thereof.

           14. Execution in Counterparts. This Amendment may be executed in any
number of counterparts, each of which shall be deemed to be an original, but all
of which taken together shall constitute one and the same instrument.


           IN WITNESS WHEREOF, the Borrower has caused this Amendment to Secured
Promissory Note to be duly executed and delivered in its name and on its behalf
as of the date first written above.

SYNBIOTICS CORPORATION

By:  /s/ Paul A. Rosinack
    ------------------------
Name:  Paul A. Rosinack
Title:  President & CEO

           The undersigned, sometimes referred to as the Holder in the foregoing
Amendment to Secured Promissory Note (the "Amendment), agrees to the amendment
of the Note (as defined in the Amendment) pursuant to the Amendment.

KIRKEGAARD & PERRY LABORATORIES, INC.

By:  /s/ Albert Perry
     ----------------------
Name:  Albert Perry
Its:  President
Date of Execution:  April 24, 2001