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                                                                     EXHIBIT 4.3

                               DONLAR CORPORATION


March 18, 2002

CONFIDENTIAL

Willis Stein & Partners, L.P.
227 West Monroe
Suite 4300
Chicago, IL  60606

Star Polymers, L.L.C.
227 West Monroe
Suite 4300
Chicago, IL  60606

Dear Sirs:

         This letter agreement (this "Letter Agreement") sets forth certain
understandings between Willis Stein & Partners, L.P. ("Willis Stein") and Star
Polymers, L.L.C. ("Star," together with Willis Stein are sometimes referred to
as the "Willis Stein Group"), and Donlar Corporation, an Illinois corporation
("Donlar"), regarding a Bridge Facility to be provided to Donlar and its
subsidiary, Donlar Biosyntrex Corporation, a Nevada corporation ("Biosyntrex")
(Donlar and Biosyntrex are referred to collectively in this Letter Agreement as
the "Company"), by Tennessee Farmers Insurance Company or one of its affiliates
("Tennessee Farmers").

         Attached to this Letter Agreement as Schedule A is the form of the
Bridge and Consolidated Term Loan Agreement (the "Loan Agreement") among Donlar,
Biosyntrex and Tennessee Farmers, which sets forth (i) the material terms and
conditions of the Bridge Facility, (ii) the restructuring of other indebtedness
of the Company to Tennessee Farmers, (iii) the restructuring of other
indebtedness of the Company, including indebtedness owed by the Company to the
Willis Stein Group and (iv) Tennessee Farmer's agreement to a merger (the
"Merger") of Donlar and Biosyntrex subject to certain conditions and on the
terms as set forth on Schedule F. Attached to this Letter Agreement as Schedule
B is a list of the indebtedness of the Company to the Willis Stein Group (the
"Willis Stein Indebtedness").

         Section 6.3 of the Loan Agreement contains the conditions to the
initial extension of credit under the Bridge Facility, including the agreement
of the Willis Stein Group to the following (capitalized terms have the same
meaning as in the Loan Agreement):

             A. exchange all the equity securities of the Company the Willis
                Stein Group holds as of the date hereof into shares of common
                stock of Donlar;

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             B. convert the Willis Stein Indebtedness into new senior
                convertible preferred stock; and

             C. execute a subordination agreement acceptable to the lenders
                under the Bridge Facility.

         In order to facilitate the completion of the Bridge Facility and the
Merger, and subject to the completion of the definitive documentation for the
foregoing transactions and, in the case of paragraphs 1, 2 and 4, the Conditions
for Conversion (defined below) being satisfied, the Willis Stein Group agrees
to:

             1. The conversion of the Willis Stein Indebtedness into shares of
                new convertible senior preferred stock (the "Senior Preferred")
                containing the terms attached hereto as Schedule C and having a
                stated liquidation value of $9 million dollars, which shares
                will be convertible into 13,235,294 shares of the Company's
                common stock and will represent 14.14% of the total outstanding
                equity of the Company on a fully diluted basis.

             2. The exchange of all the equity securities of the Company the
                Willis Stein Group holds into one (1) million shares of common
                stock, which shares will represent 1.07% of the total
                outstanding equity of the Company on a fully diluted basis.

             3. Execute the Subordination Agreement in the form of Schedule H.

             4. Vote any shares of voting securities of the Company owned by the
                Willis Stein Group in favor of the Merger.

             5. Subject to the terms and conditions contained herein, execute
                and deliver such further instruments, certificates or documents
                and take such further action as may be reasonably necessary to
                more fully carry out the intent and the purposes of paragraphs 1
                and 2.

             6. Acknowledge that Tennessee Farmers is an express third party
                beneficiary of this Letter Agreement and shall have all of the
                claims, rights, powers and remedies of such a third party
                beneficiary to the maximum extent permitted under applicable
                law.

         The obligations of the Willis Stein Group set forth in paragraphs 1, 2
and 4 above are subject to the following conditions ("Conditions for
Conversion") being met:

                (i)    the amendment to the Company's Certificate of
                       Incorporation (the "Amendment") shall contain the terms
                       of the Senior Preferred described in Schedule C and no
                       other terms that are inconsistent with those set forth on
                       Schedule C, or otherwise detrimental to the holders of
                       Senior Preferred;

                (ii)   the Company shall have received all requisite consents
                       and approvals to the transactions described herein,
                       including receiving approval from its

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                       board of directors and shareholders for filing the
                       Amendment; and the Company shall have duly executed and
                       filed with the Secretary of State of Illinois the
                       Amendment; and such Amendment shall be in full force and
                       effect at the time of conversion;

                (iii)  any and all indebtedness owing to, and all equity of, Dr.
                       Gale Martin shall have been exchanged for: (a) shares of
                       Senior Preferred having a stated liquidation value of
                       nine (9) million dollars, which shares shall be
                       convertible into for 13,235,294 shares of common stock;
                       (b) five (5) million shares of common stock (the "Common
                       Stock"); and (c) a warrant for an additional three (3)
                       million shares of common stock (the "Warrant"). The
                       Senior Preferred, Common Stock and Warrant issued to Dr.
                       Gale Martin will represent 22.68% of the total
                       outstanding equity of the Company on a fully diluted
                       basis;

                (iv)   the Company and Tennessee Farmers shall have executed
                       definitive documents which are consistent with the Loan
                       Agreement set forth in Schedule A hereto and Tennessee
                       Farmers has committed to lend approximately $2,127,000 to
                       the Company in accordance therewith;

                (v)    Donlar, Biosyntrex and Tennessee Farmers enter into an
                       agreement regarding the restructuring of the Company
                       consistent with the terms set forth in Schedule F hereto;

                (vi)   the 1998 and 2000 notes set forth on Schedule G shall be
                       simultaneously exchanged for shares of Senior Preferred
                       having a stated liquidation value of $1,905,500 which
                       shares will be convertible into 2,802,206 shares of
                       common stock;

                (vii)  the capitalization of the Company on a fully diluted
                       basis immediately prior to the Merger is as set forth on
                       Schedule D and the capitalization of the Company on a
                       fully diluted basis upon consummation of the Merger shall
                       be as set forth on Schedule E; and

                (viii) the Company shall have represented and warranted to the
                       Willis Stein Group that all of the Conditions for
                       Conversion have been satisfied.

         If the foregoing is in accordance with your understanding of our
agreement, please sign where indicated below and deliver a copy of this Letter
Agreement as provided for herein, whereupon this Letter Agreement shall
represent a binding agreement between us and shall be governed by the internal
laws of the State of Illinois.

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                                         Very truly yours,

                                         DONLAR CORPORATION

                                         By:
                                            ---------------------------------
                                               Name:  Larry Koskan
                                               Title: President and CEO



Accepted and agreed to as of
the date first above written:

WILLIS STEIN & PARTNERS, L.P.

By:
   ---------------------------------
      Name:
      Title:

STAR POLYMERS, L.L.C.


By:
   ---------------------------------
      Name:
      Title: