Document:

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                                                                 Exhibit 10.51.4

                       FIRST AMENDMENT TO CREDIT AGREEMENT

      Reference is hereby made to that certain Credit Agreement dated as of
December 28, 1998, by and between Genzyme Transgenics Corporation (the
"Borrower"), and Fleet National Bank ("Lender") (as amended, and as may be
further amended from time to time, the "Credit Agreement"). Capitalized terms
not defined herein shall have the meanings ascribed thereto in the Credit
Agreement.

      WHEREAS, the Borrower and Guarantor have requested that the Lender consent
to an increase the maximum amount of Dividends that may be paid to holders of
its Series A Convertible Preferred Stock for the redemption of such stock and
accrued Dividends thereon, and to agree to permit the payment of Dividends up to
a maximum amount for the purpose of the redemption of Series B Convertible
Preferred Stock and accrued Dividends thereon, and the Lender has agreed to such
change, subject to the terms and conditions of this First Amendment To Credit
Agreement (the "First Amendment").

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

1. Dividends and Redemptions. The Credit Agreement is hereby amended as follows:

            a. Section 2.5 (Revolving Credit Loan Proceeds) of the Credit
Agreement is hereby amended by replacing the phrase "or to redeem shares of its
Series A Convertible Preferred Stock, face value $1,000 per share" with the
phrase "or to redeem shares of its Series B Convertible Preferred Stock, face
value $1,000 per share."

            b. Section 6.6 (Dividends and Redemptions) of the Credit Agreement
is hereby deleted and replaced with the following in lieu thereof:

            "6.6 Dividends and Redemptions. Pay or set aside any amount to pay
            any Dividends; provided, however, that, so long as no Default or
            Event of Default has occurred and is continuing, Borrower may pay
            Dividends to holders of its Series B Convertible Preferred Stock for
            the redemption of such stock and accrued Dividends payable in
            connection with such stock, up to a maximum amount of $15,000,000,
            in accordance with Schedule 6.6 which sets forth the payment
            schedule for the redemption of such stock."

            c. Schedule 6.6 to the Credit Agreement is hereby amended to replace
the sentence "The entire $11,500,000 could be used at any time to repurchase the
Series A Convertible Stock" as it appears therein the following sentence in lieu
thereof: "The entire $15,000,000 could be used at any time to repurchase the
Series B Convertible Stock"; and to replace the title thereof: "Series A
Convertible Stock Purchase Schedule" with the following title in lieu thereof:
"Series B Convertible Stock Purchase Schedule."

2. Consent. The Borrower and Guarantor have requested that the Lender consent to
the Borrower's payment of Dividends to holders of the Borrower's Series A
Convertible Preferred

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Stock for the redemption of such stock and accrued Dividends payable in
connection with such stock, up to a maximum amount of $12,650,000 (the "November
Redemption Transaction"). The funds to be used for such payments will consist of
the entire purchase price to be paid by the Guarantor to purchase Series B
Convertible Preferred Stock of the Borrower, up to a maximum of $12,500,000,
and, if the November Redemption Transaction requires total payments exceeding
$12,500,000, the remaining required amount of such payments will be provided by
the Borrower, up to a maximum amount of $150,000. The Lender's consent to the
payments contemplated by the November Redemption Transaction is required because
(a) prior to the effectiveness of this Amendment, Section 6.6 of the Credit
Agreement limits the total amount of such payments to $11,500,000, and the
maximum that the total aggregate payments required by the November Redemption
Transaction could be is $12,650,000, and (b) after the effectiveness of this
First Amendment, the Borrower may not make any payments of Dividends on account
of Series A Convertible Preferred Stock. The Lender hereby consents to the
Borrower's making such payments in connection with the November Redemption
Transaction.

3. Representations and Warranties. In order to induce the Lender to enter into
this First Amendment, the Borrower and Guarantor make the following
representations and warranties, all of which shall survive the execution and
delivery of this First Amendment:

      (a) Each of the Borrower and the Guarantor has all requisite corporate,
partnership or other power and authority to execute, deliver and perform its
obligations under this First Amendment and under the Credit Agreement, as
amended hereby. This First Amendment has been duly authorized, executed and
delivered by the Borrower and the Guarantor, and does not conflict with, violate
or result in a breach of or require any consent under any applicable law, rule
or regulation or any of the terms of the charter or by-laws (or equivalent
constitutional documents) of the Borrower or the Guarantor, any agreement or
instrument to which the Company, the Guarantor or any Subsidiary of either of
them is a party or to which any of them or their property is bound or to which
any of them is subject. This First Amendment and the Credit Agreement, as
amended hereby, constitute the legal, valid and binding obligation of the
Borrower and the Guarantor enforceable against each in accordance with its
terms.

      (b) On the date hereof each of the representations and warranties in the
Credit Agreement are true, accurate and complete in all material respects,
provided that, if any representation or warranty is expressly required in the
Credit Agreement to be made only as of a specific date, such representation or
warranty shall be true, accurate and complete as of such date in all material
respects.

      (c) Upon the execution and delivery of this First Amendment, and the
satisfaction of each of the conditions precedent set forth in Section 4 of this
First Amendment, no Default or Event of Default shall exist and be continuing.

4. Conditions Precedent. The agreements contained herein and the amendments
contemplated hereby shall become effective on the date (the "Effective Date")
when the Borrower, the Guarantor, and the Lender shall have executed this First
Amendment and when each of the following conditions shall have been fulfilled:

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      (a) Execution of Documents, Etc. This First Amendment and any other
agreements, documents and instruments to be executed and/or delivered in
connection herewith (collectively the "First Amendment Documents") shall have
been duly and properly authorized and executed by the Borrower, the Guarantor,
and the Lender and shall be in full force and effect on and as of the Effective
Date of this First Amendment and all representations and warranties of the
Borrower and Guarantor hereunder shall continue to be true, accurate and
complete.

      (b) Proceedings; Receipt of Documents. All requisite corporate action and
proceedings of the Borrower and Guarantor in connection with the execution and
delivery of this First Amendment and the other First Amendment Documents shall
be satisfactory in form and substance to the Lender and its counsel, and the
Lender and its counsel shall have received all information and copies of all
documents, including without limitation, records of requisite corporate action
and proceedings which the Lender or its counsel may have requested in connection
therewith, such documents where requested by the Lender or its counsel to be
certified by appropriate persons or governmental authorities.

      (c) Material Litigation. There shall be no pending or, to the best
knowledge of the Borrower, threatened litigation with respect to the Borrower or
the Guarantor before any court, arbitrator or governmental or administrative
body or agency which challenges or relates to (i) the transactions contemplated
hereby or (ii) the Loan Documents.

5. Reaffirmation and Ratification of Existing Agreements, Etc. Each of the
Borrower and the Guarantor: (i) reaffirms and ratifies all the Obligations to
the Lender, in respect of the Credit Agreement, as hereby amended, and the other
Loan Documents, (ii) certifies that there are no defenses, offsets or
counterclaims to such Obligations as of the date hereof , (iii) expressly
acknowledges its continuing liability pursuant thereto, and (iv) agrees that
each of the Credit Agreement, as amended hereby, and the other Loan Documents
shall remain in full force and effect, enforceable against the Borrower and
Guarantor in accordance with its terms.

6. Miscellaneous.

      (a) This First Amendment may be executed on separate counterparts by the
parties hereto, each of which when so executed and delivered shall be an
original, but all of which shall constitute one and the same agreement.

      (b) This First Amendment and the rights and obligations of the parties
hereunder shall be construed in accordance with and be governed by the laws of
the Commonwealth of Massachusetts (without giving effect to the conflict of law
principles thereof).

      (c) The headings of the several sections of this First Amendment are
inserted for convenience only and shall not in any way affect the meaning or
construction of any provision of this First Amendment.

      (d) This First Amendment, together with the other First Amendment
Documents, embodies the entire agreement and understanding among the parties
relating to the subject matter

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hereof and supersedes all prior proposals, negotiation, agreements and
understandings relating to such subject matter.

      (e) This First Amendment, together with the other First Amendment
Documents, shall be deemed to be Loan Documents under the Credit Agreement.

      (f) EACH OF THE BORROWER, THE GUARANTOR, AND THE LENDER HEREBY IRREVOCABLY
WAIVES, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, ANY AND ALL RIGHT TO
TRIAL BY JURY IN ANY LEGAL PROCEEDING ARISING OUT OF OR RELATING TO THIS FIRST
AMENDMENT, OR THE TRANSACTIONS CONTEMPLATED HEREBY.

      (g) The Borrower shall pay on demand the reasonable costs and expenses,
including, without limitation, reasonable attorneys' fees and expenses incurred,
or which may be incurred by the Lender in connection with the negotiation,
documentation, administration and enforcement of this First Amendment.

      IN WITNESS WHEREOF, this First Amendment has been duly executed and
delivered as a sealed instrument at Boston, Massachusetts as of this 12th day of
November, 1999.

                                    THE BORROWER:

                                    GENZYME TRANSGENICS CORPORATION

                                    By: /s/ John B. Green
                                       ----------------------------
                                       Title:

                                    THE GUARANTOR:

                                    GENZYME CORPORATION

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

                                    THE LENDER:

                                    FLEET NATIONAL BANK

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

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hereof and supersedes all prior proposals, negotiation, agreements and
understandings relating to such subject matter.

      (e) This First Amendment, together with the other First Amendment
Documents, shall be deemed to be Loan Documents under the Credit Agreement.

      (f) EACH OF THE BORROWER, THE GUARANTOR, AND THE LENDER HEREBY IRREVOCABLY
WAIVES, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, ANY AND ALL RIGHT TO
TRIAL BY JURY IN ANY LEGAL PROCEEDING ARISING OUT OF OR RELATING TO THIS FIRST
AMENDMENT, OR THE TRANSACTIONS CONTEMPLATED HEREBY.

      (g) The Borrower shall pay on demand the reasonable costs and expenses,
including, without limitation, reasonable attorneys' fees and expenses incurred,
or which may be incurred by the Lender in connection with the negotiation,
documentation, administration and enforcement of this First Amendment.

      IN WITNESS WHEREOF, this First Amendment has been duly executed and
delivered as a sealed instrument at Boston, Massachusetts as of this 12th day of
November, 1999.

                                    THE BORROWER:

                                    GENZYME TRANSGENICS CORPORATION

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

                                    THE GUARANTOR:

                                    GENZYME CORPORATION

                                    By: /s/ Michael S. Wyzga
                                       ----------------------------
                                       Title:

                                    THE LENDER:

                                    FLEET NATIONAL BANK

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

<PAGE>

hereof and supersedes all prior proposals, negotiation, agreements and
understandings relating to such subject matter.

      (e) This First Amendment, together with the other First Amendment
Documents, shall be deemed to be Loan Documents under the Credit Agreement.

      (f) EACH OF THE BORROWER, THE GUARANTOR, AND THE LENDER HEREBY IRREVOCABLY
WAIVES, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, ANY AND ALL RIGHT TO
TRIAL BY JURY IN ANY LEGAL PROCEEDING ARISING OUT OF OR RELATING TO THIS FIRST
AMENDMENT, OR THE TRANSACTIONS CONTEMPLATED HEREBY.

      (g) The Borrower shall pay on demand the reasonable costs and expenses,
including, without limitation, reasonable attorneys' fees and expenses incurred,
or which may be incurred by the Lender in connection with the negotiation,
documentation, administration and enforcement of this First Amendment.

      IN WITNESS WHEREOF, this First Amendment has been duly executed and
delivered as a sealed instrument at Boston, Massachusetts as of this 12th day of
November, 1999.

                                    THE BORROWER:

                                    GENZYME TRANSGENICS CORPORATION

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

                                    THE GUARANTOR:

                                    GENZYME CORPORATION

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

                                    THE LENDER:

                                    FLEET NATIONAL BANK

                                    By: /s/ Kimberly Martone
                                       ----------------------------
                                       Title: SVP<PAGE>

July 20, 1999

MCNIC Pipeline & Processing Company
150 West Jefferson Avenue
Suite 1700
Detroit, MI 48226

Re:      Crown Asphalt Ridge, L.L.C.
         Crown Asphalt Distribution Two, L.L.C.

Ladies and Gentlemen:

         This letter is written to evidence our agreement made in conjunction
with the Loan Agreement and Promissory Note (the "FINANCING DOCUMENTS") entered
into today between Crown Asphalt Corporation ("CAC") and MCNIC Pipeline &
Processing Company ("MCNIC").

1. The Financing Documents and subject loan transaction were entered into such
that the loan proceeds will be used to satisfy the Secondary Capital
Contributions required, as of the date of the Financing Documents, of CAC under
Section 3.3 of the Operating Agreement (the "CAR OPERATING AGREEMENT") for Crown
Asphalt Ridge, L.L.C. ("CAR") dated as of September 1, 1997. The net cash
proceeds of the loan made under the Financing Documents ($1,891,650.50), after
deduction of amounts previously paid by MCNIC to creditors of CAR and less
certain amounts owed by CAR and/or CAC to MCNIC, shall be paid by MCNIC directly
to CAR. The gross proceeds of the loan ($2,991,868.66) made under the Financing
Documents shall be deemed the contribution by CAC to CAR, following which CAC
shall have a 25% Sharing Ratio and MCNIC shall have a 75% Sharing Ratio in CAR.
Of course, additional capital contributions may be required in the future as
otherwise provided under the CAR Operating Agreement.

2. We have been negotiating the terms of a Contribution Agreement between Crown
Asphalt Products Company ("CAPCO") and MCNIC (the "CONTRIBUTION AGREEMENT," the
most recent draft of which is attached hereto as EXHIBIT A). The Contribution
Agreement sets forth, among other things, the manner in which

(a) Crown Asphalt Distribution Two, L.L.C. ("CAD TWO") will be formed,

(b) CAD Two will acquire the Laurel, Montana asphalt terminal and the Williston,
North Dakota asphalt terminal (collectively, the "ASA TERMINALS") pursuant to
the terms of the existing Asphalt Supply Agreement (as defined in such
Contribution Agreement), AND

(c) MCNIC will make its required capital contributions related to the ASA
Terminals under such Contribution Agreement.

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When each of the actions or events specified in Section 2, subsections (a), (b)
and (c), have been completed, the ASA Terminals will have been acquired by CAD
Two as contemplated by the Contribution Agreement and for, convenience of
reference, we will refer to this CAD Two acquisition as the "ORIGINAL ASPHALT
SUPPLY ARRANGEMENT."

         If the terms of the Asphalt Supply Agreement are materially changed or
modified in order to finally complete (including payment of the purchase price
to the seller) the Asphalt Supply Agreement, we will refer to these amended
acquisition terms as a "REVISED ASPHALT SUPPLY ARRANGEMENT."

3. This letter further evidences our agreements regarding the performance of the
Contribution Agreement depending on if, how and when the Asphalt Supply
Agreement is completed:

         (a) If the acquisition of the ASA Terminals is completed under the
Original Asphalt Supply Arrangement on or before August 20, 1999, the
Contribution Agreement shall be performed in accordance with its terms.

         (b) If the acquisition of the ASA Terminals is completed under the
Original Asphalt Supply Arrangement after August 20, 1999, the ASA Terminals
business deal shall constitute an "additional opportunity" under Section 6.3 of
the initial Operating Agreement (prior to and excluding any amendment on or
after the date hereof) of Crown Asphalt Distribution, L.L.C. ("CAD").

         (c) If the acquisition of the ASA Terminals is completed under a
Revised Asphalt Supply Arrangement, the ASA Terminals business deal shall
constitute an "additional opportunity" under amended Section 6.3 of the CAD
Operating Agreement (as amended pursuant to a First Amendment to Operating
Agreement executed in connection with this letter agreement).

4. Notwithstanding anything to the contrary in Section 6.3 of the initial CAD
Operating Agreement (prior to and excluding any amendment on or after the date
hereof), if the acquisition of the ASA Terminals is completed under the Original
Asphalt Supply Arrangement (and, after August 20, 1999, if MCNIC elects to
participate), then the Contribution Agreement, an Operating Agreement in
substantially the same form attached hereto as EXHIBIT B and an Operating and
Management Agreement substantially similar to that executed in connection with
CAD (collectively referred to as the "CAD TWO AGREEMENTS") shall be executed and
delivered by the parties AND CAPCO will further secure the loan evidenced by the
Financing Documents with its interests in CAD by causing the execution of a
Guaranty and Security Agreement in substantially the same form as those
attached, respectively, as EXHIBITS C AND D.

5. Conversely, if the ASA Terminals are acquired under a Revised Asphalt Supply
Arrangement and MCNIC elects to participate in the "additional opportunity" in
accordance requirements of Section 3(c) above, then either:

         (a) if CAPCO and MCNIC mutually agree, in their respective and sole
discretion, upon amended terms to acquire the ASA Terminals and the revised
structure of their respective contributions to CAD Two with respect to the ASA
Terminals and the Rawlins, Wyoming

<PAGE>

terminal which was acquired by CAPCO pursuant to the S&L Agreement (as defined
in the Contribution Agreement), then (i) the Contribution Agreement and the CAD
Two Agreements shall be revised to reflect such amended terms and the Revised
Asphalt Supply Arrangement and (ii) MCNIC and CAPCO shall execute and deliver
such agreements; OR

(b) if CAPCO and MCNIC do not mutually agree as described in Section 5(a) above,
either CAPCO or MCNIC may provide a written request for final determination to
the other party and, if CAPCO and MCNIC do not then mutually agree within ten
(10) days after the date of such request, then (i) CAPCO will contribute to CAD
Two only the Rawlins, Wyoming terminal pursuant to the S & L Agreement (as
defined in the Contribution Agreement), (ii) the Contribution Agreement and the
CAD Two Agreements shall be revised to delete all terms directly related to the
assignment, assumption, funding or other performance of the Asphalt Supply
Agreement and MCNIC and CAPCO shall execute, deliver and perform such
agreements, and (iii) upon contribution of the Rawlins, Wyoming terminal to CAD
Two pursuant to the Contribution Agreement, MCNIC shall unconditionally
guarantee and be liable for one-half of all obligations, debt and other
liabilities of CAPCO and Crown Energy Corporation arising from the original
acquisition under the S & L Agreement and shall execute and deliver such further
agreements or instruments as may be reasonable required to confirm the same.

Please indicate your agreement with the foregoing terms by executing a copy of
this letter in the space indicated below.

Very truly yours,

CROWN ASPHALT CORPORATION

By:
  ----------------------------------
      Jay Mealey, President

CROWN ASPHALT PRODUCTS COMPANY

By:
  ----------------------------------
      Jay Mealey, President

Accepted and agreed to this 20th day of July, 1999.

MCNIC PIPELINE & PROCESSING COMPANY

By:
   ----------------------------------------
      William E. Kraemer, Vice President

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