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                                                                   Exhibit 10.43

                      FIRST AMENDMENT TO LICENSE AGREEMENT

     This First Amendment is made and entered into as of the thirteenth day of
December, 1999, by and between HESKA CORPORATION, a Delaware corporation having
its principal place of business at 1613 Prospect Parkway, Fort Collins, Colorado
80525 (Facsimile: 970-484-9505) ("HESKA") and IMMULOGIC PHARMACEUTICAL
CORPORATION, a Delaware corporation having its principal place of business at
610 Lincoln Street, Waltham, Massachusetts 02154 (Facsimile: 781-466-6050)
("IMMULOGIC").

     WHEREAS, HESKA and IMMULOGIC are parties to that certain License Agreement
dated as of June 16, 1998 (the "LICENCE AGREEMENT"); and

     WHEREAS, the Licensed Technology described in EXHIBIT A attached hereto is
owned by IMMULOGIC (the "OWNED TECHNOLOGY") and consists of (i) the patent and
patent applications assigned solely to IMMULOGIC and listed on Exhibit A-1 and
(ii) IMMULOGIC's ownership rights in the patents and patent applications
assigned to IMMULOGIC and another party and listed on Exhibit A-2; and

     WHEREAS, IMMULOGIC owns the ALLERVAX(TM) trademark and all registrations
and goodwill associated therewith (also referred to herein as part of the "Owned
Technology"); and

     WHEREAS, IMMULOGIC and Sankyo Co. Ltd. Are parties to that certain Patent
License Agreement dated as of May 22, 1998, a copy of which is attached as
Exhibit E hereto (the "Sankyo License"); and

     WHEREAS, the Licensed Technology described in EXHIBIT B attached hereto is
licensed by IMMULOGIC from third parties and sublicensed to HESKA pursuant to
the License Agreement ("SUBLICENSED TECHNOLOGY") and consists of (i)) the patent
and patent applications assigned to another party and licensed exclusively to
IMMULOGIC and listed on Exhibit B-1 and (ii) IMMULOGIC's exclusive license
rights in the patents and patent applications assigned to IMMULOGIC and another
party and listed on Exhibit B-2; and

     WHEREAS, HESKA and IMMULOGIC desire to amend the License Agreement as set
forth in this First Amendment;

     NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the parties hereto agree as
follows:

     1.   DEFINITIONS. Capitalized terms used in this First Amendment (and the
Recitals above) shall have the meanings ascribed to them in the License
Agreement, unless otherwise expressly defined herein.

     2.   AMENDMENTS TO LICENSE AGREEMENT. Upon execution and delivery of this
First Amendment, HESKA shall pay to IMMULOGIC Three Hundred Thousand Dollars
($300,000), in immediately available funds (the "BUYOUT PAYMENT"). In
consideration of the Buyout Payment, the License Agreement is hereby amended as
follows:

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          (a)  CONVEYANCE OF OWNED TECHN0LOGY TO HESKA.

               (i)  IMMULOGIC hereby conveys, transfers and sets over to HESKA
all right title and interest in and to the Owned Technology. IMMULOGIC hereby
agrees, at its own expense, to do all such things and take all such actions, and
to make, execute and deliver all such other documents and instruments, including
but not limited to patent assignments, trademark assignments, copyright
assignments, bills of sale, and other conveyance documents, as shall be
reasonably requested by HESKA to implement the foregoing conveyance of the Owned
Technology and otherwise carry out the provisions, intent and purpose of this
First Amendment.

               (ii) IMMULOGIC hereby represents and warrants to HESKA that
(A) IMMULOGIC has, and conveys to HESKA hereunder, good and marketable title to
the Owned Technology, free and clear of all liens, encumbrances and restrictions
of any kind whatsoever (other than as expressly set forth in EXHIBIT A) and (B)
the representations and warranties of IMMULOGIC set forth in Sections 7.1(b),
(e), (f) and (g) of the License Agreement are true and correct with respect to
the Owned Technology and this First Amendment as if made on the date of this
First Amendment.

          (b)  ASSIGNMENT AND ASSUMPTION OF SUBLICENSED TECHNOLOGY.

               (i)  Promptly after the execution and delivery of this First
Amendment, but in no event after June 30, 2000, IMMULOGIC shall cause each
licensor of the Sublicensed Technology to execute and deliver to HESKA an
Assignment and Assumption Agreement in the form of EXHIBIT C attached hereto,
which shall incorporate the terms and conditions set forth in EXHIBIT D attached
hereto relating to each respective licensor (each, an "Assistant Agreement").

               (ii) If IMMULOGIC shall fail to deliver any Assignment Agreement
when due, HESKA may exercise any and all of its rights and remedies for breach
of the License Agreement, and without limiting the generality of the foregoing,
if IMMULOGIC shall fail to deliver all Assignment Agreements on or before the
first date that a milestone payment is otherwise due and payable under Section
3.1(b)(ii) or Section 3.1(b)(iii) of the License Agreement, HESKA may, by
written notice to IMMULOGIC, terminate its obligations under such Sections
3.1(b)(ii) and 3.1(b)(iii).

               (iii) If IMMULOGIC in good faith seeks to obtain an Assignment
Agreement from any licensor and such licensor refuses in writing to enter into
such Assignment Agreement due to any provisions of the applicable Exhibit D,
IMMULOGIC shall be released from its obligation to deliver the Assignment
Agreement unless HESKA agrees to waive such provision (but not any other
provision) of Exhibit D to which the licensor will not agree within 10 days of
written notice from IMMULOGIC. In the event IMMULOGIC is released from its
obligation to deliver a particular Assignment Agreement under this subparagraph,
the provisions of section 2(b)(ii) shall not apply with respect to that
Assignment Agreement.

          (c)  ELIMINATION OF ROYALTIES AND CERTAIN MILESTONE PAYMENTS. The
following sections of the License Agreement are hereby deleted in their entirety
and shall have no further force and effect: Section 3.1(b)(i), Section 3.2(a),
Section 3.2(b), Section 3.2(c), Section 3.3, Section 3.4(a) and Section 3.4(c).
Without limiting the generality of the foregoing, no further

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consideration shall be payable by HESKA to IMMULOGIC under the License Agreement
except for the amounts payable, if any, pursuant to the terms and conditions of
Section 3.1(b)(ii) and Section 3.1(b)(iii). Article 4 of the License Agreement
shall apply only with respect to payments, if any, due under Section 3.1(b)(ii)
and Section 3.1(b)(iii).

          (d)  COVENANT NOT TO COMPETE. Article 5 of the License Agreement shall
continue to apply to the Owned Technology and Sublicensed Technology, for a
period of five (5) years after the date of this First Amendment. The last
sentence of Article 5 of the License Agreement is hereby deleted in its
entirety.

          (e)  PATENT PROSECUTION: INFRINGEMENT ACTIONS. IMMULOGIC's rights and
obligations under Article 6 of the License Agreement with respect to the Owned
Technology shall be of no further force and effect after the date of this First
Amendment.

          (f)  ASSUMPTION OF SANKYO LICENSE. IMMULOGIC hereby conveys and
assigns to HESKA, as of the date hereof, all of IMMULOGIC's right, title and
interest in and to the Sankyo License, and HESKA hereby assumes the Sankyo
License as licensor thereunder and agree to perform all obligations to be
performed thereunder on and after such date (but not other obligations).
IMMULOGIC represents and warrants that IMMULOGIC has performed all of its
obligations under the Sankyo License through the date hereof.

     3.   EFFECT OF THIS FIRST AMENDMENT. In the event of any conflict between
the terms and conditions of this First Amendment and the License Agreement, this
First Amendment shall govern and control. Upon execution and delivery of this
First Amendment, the License Agreement shall continue in full force and effect,
as amended by the express terms of this First Amendment.


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     IN WITNESS WHEREOF, the parties have executed this First Amendment by their
duly authorized representatives, effective as of the date on which this First
Amendment has been duly signed by both parties, which date will be inserted at
the top of this Agreement.

                                    IMMULOGIC PHARMACEUTICAL
                                    CORPORATION

                                    By /s/ Richard Crowley
                                       ------------------------------
                                       Richard Crowley
                                       President

                                    HESKA CORPORATION

                                    By /s/ Paul Hudnut
                                       ------------------------------
                                       Paul Hudnut
                                       Executive Vice President

EXHIBITS:
--------

A        Owned Technology
B        Sublicensed Technology
C        Form of Assignment and Assumption Agreement
D        Individual Terms and Conditions of Each Assignment Agreement
E        Sankyo License


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                                    EXHIBIT A

                    TO FIRST AMENDMENT TO LICENSE AGREEMENT

                                Owned Technology

PATENT AND PATENT ASSIGNMENTS

The patents and patent applications listed on Exhibit A-1 and IMMULOGIC's
ownership rights in the patents and patent applications assigned to IMMULOGIC
and another party and listed on Exhibit A-2.

TRADEMARKS

ALLERVAX

LIENS AND ENCUMBRANCES

Patent License Agreement, dated May 22, 1998 between ImmuLogic Pharmaceutical
Corporation and Sankyo Co. Ltd., a correct and complete copy of which is
attached to this First Amendment to License Agreement as Exhibit E.

KNOW-HOW AND BIOLOGICAL MATERIALS

As stated in the License Agreement and its exhibits.


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