<PAGE>

Portions of this exhibit have been omitted and separately filed with the
Commission pursuant to a request for confidential treatment under Rule 24b-2.
The location of those omissions is denoted by opening and closing brackets as
follows: [ ].

                                                                   Exhibit 10.38
                                    GUARANTY

     This GUARANTY (the "Guaranty") is made this 15 day of December, 2000, by
THE GENESEE BREWING COMPANY, INC., a New York corporation ("Genesee"), in favor
of BOSTON BREWING COMPANY, INC., d/b/a The Boston Beer Company, a Massachusetts
corporation, for itself and as the sole general partner of Boston Beer Company
Limited Partnership, a Massachusetts limited partnership (collectively, "Boston
Brewing").

                                R E C I T A L S:

     WHEREAS, Genesee and Boston Brewing are parties to an Amended and Restated
Agreement dated as of April 30, 1997 (the "Production Agreement"), under which
Genesee has agreed to supply Boston Brewing, and Boston Brewing has agreed to
purchase from Genesee, on an as-ordered basis, Samuel Adams Boston Lager and
certain other Boston Brewing products;

     WHEREAS, Genesee and Monroe Brewing Co., LLC ("Monroe Brewing") have
entered into an Asset Purchase Agreement dated of August 29, 2000, as amended by
Amendment No. 1 dated as of December 15, 2000 (collectively, the "Purchase
Agreement"), pursuant to which Genesee has agreed to sell to Monroe, and Monroe
has agreed to purchase from Genesee, substantially all of the assets necessary
to operate Genesee's brewing business, including, but not limited to, all of
Genesee's right, title and interest in and to the Production Agreement; and

     WHEREAS, at Genesee's request, Boston Brewing has consented to the transfer
of the Production Agreement from Genesee to Monroe Brewing pursuant to a certain
Consent dated December 15, 2000 (the "Consent"), which conditions the
effectiveness of consent on Genesee's execution and delivery to Boston Brewing
of a guaranty in favor of Boston Brewing substantially in the form hereof;

                              P R O V I S I O N S:

     NOW, THEREFORE, in consideration of the Consent provided by Boston Brewing
and other good and valuable consideration, Genesee hereby agrees in favor of
Boston Brewing as follows:

     1. Performance Guaranty.

        (a) Subject to the terms and conditions herein, Genesee hereby
guarantees the due, punctual and faithful performance by Monroe Brewing of its
obligations, duties, covenants and responsibilities under the Production
Agreement that arise after the closing under the Purchase Agreement based on
facts occurring subsequent thereto. Genesee hereby specifically further agrees
that in the event Monroe Brewing, for any reason, is obligated to perform under
the Production Agreement and it fails to do so in accordance with the terms
thereof, Genesee will, upon written notice from Boston Brewing, become liable to
Boston Brewing for any sum or sums Monroe Brewing has to pay Boston Brewing
under the terms of the Production Agreement
<PAGE>

for failure to perform or pay, subject to the terms and conditions contained
therein or any defenses thereto which either Monroe Brewing may have or which
Genesee may have.

        (b) Notwithstanding anything herein to the contrary, Genesee's
obligations under this Guaranty and the Production Agreement shall terminate and
be of no further force or effect upon the expiration of 1,095 days after the
"Closing Date" (as defined in the Purchase Agreement ) (the "Termnation Date"),
provided that any claims made in writing by Boston Brewing against Genesee
hereunder prior to the Termination Date and remaining unresolved or outstanding
as of such date shall survive until the earlier to occur of the payment or the
final resolution and satisfaction of such claims to the extent and subject to
the terms and conditions herein. The period between the Closing Date and the
Termination Date is referred to herein as the "Guaranty Period."

     2. Net Worth Maintenance.

        (a) During the Guaranty Period, Genesee will maintain a Liquid Net Worth
(defined for this purpose as the excess of (i) unencumbered (1) cash and net
currency on hand and on deposit, demand deposits and checks held, (2)
short-term, highly liquid investments that are readily convertible to known
amounts of cash plus (3) marketable securities over (ii) total liabilities, each
as determined in accordance with generally acceptable accounting practices), in
an amount that, in the aggregate, is not less than the following:

           First 365 days after Closing Date:         [$          ]
           Second 365 days after Closing Date         [$          ]
           Third 365 days after Closing Date          [$          ]

        (b) After the Guaranty Period, Genesee will maintain a Liquid Net Worth
in an amount that, in the aggregate, is not less than the amount of any claims
made by Boston Brewing in accordance with Section 1 hereof that remain
unresolved.

3. Enforcement / Notice.

        (a) If Monroe Brewing fails to perform any of its duties under the
Production Agreement guaranteed hereby, then Boston Brewing may proceed directly
to make a claim against Genesee to be paid by Genesee without necessity of any
suit or proceeding by Boston Brewing against Monroe Brewing. Genesee shall be
entitled to assert any defenses or claims that Genesee or Monroe Brewing may
have under the Production Agreement or otherwise against any claim made by
Boston Brewing and does not otherwise waive any defenses whatsoever to its
obligations hereunder or under the Production Agreement. Anytime, with or
without consideration or notice, Boston Brewing may waive enforcement of the
terms, conditions and provisions of this Guaranty with respect to any breach or
default by Monroe Brewing, and such waiver will not diminish or otherwise affect
Monroe Brewing's obligations to Boston Brewing under the Production Agreement.
Genesee agrees that in the event any of the foregoing provisions are found to be
unenforceable, that portion so found will in no way affect the purpose and
intent of the remaining provisions, and to the extent those provisions will
remain binding upon the parties.
<PAGE>

        (b) Any claim, notice or other communication under this Guaranty shall
be in writing, and shall be deemed duly given when delivered personally or by
facsimile, or four days after being mailed by registered mail, return receipt
requested, or by documented over-night delivery to a party at the following
address (or such other address as such party may have specified by notice given
to the other party pursuant to this provision):

              If to Genesee, to:

                    Mark W. Leunig, Vice President
                    The Genesee Corporation
                    P.O. Box 762
                    Rochester, New York 14603

              with a copy to:

                    Monroe Brewing Co., LLC
                    445 St. Paul Street
                    Rochester, New York 14605
                    Attention: Samuel T. Hubbard, Jr., President

              If to Boston Brewing, to:

                    c/o The Boston Beer Company
                    75 Arlington Street
                    Boston, MA 02116
                    Attention: Martin F. Roper, President

     4. Subrogation. Genesee shall be entitled to, and shall be fully subrogated
to all of Boston Brewing's rights under the Production Agreement with respect
to, and to the extent of, payments made by Genesee pursuant to this Guaranty,
and Boston Brewing hereby assigns any rights that may arise in connection with
such payment to enforce any remedy which Boston Brewing may have against Monroe
Brewing therefor.

     5. Amendments and Modification to Production Agreement. As a condition to
Genesee's continuing liability hereunder, by its acceptance hereof, Boston
Brewing hereby acknowledges that Monroe Brewing has agreed with Genesee that, so
long as Genesee has any liability to Boston Brewing, it shall not agree to amend
or modify the Production Agreement, or to accept any funding, advances or other
monies or credit from Boston Brewing, in any manner that would in any way
increase or enlarge Genesee's duties, obligations or liabilities Boston Brewing
without Genesee's prior written consent. Accordingly, no such amendment or
modification shall be effective unless it is accompanied or preceded by
Genesee's written consent.
<PAGE>

     6. Representatives and Warranties.

        (a) Genesee has the corporate power and authority and legal right to
execute and deliver this Guaranty, perform its obligations hereunder and
consumate the transactions herein contemplated. The execution and delivery by
Genesee of this Guaranty and the performance of its obligations and consummation
of the transactions contemplated hereunder have been duly authorized by proper
corporate proceedings, and this Guaranty constitutes the legal, valid and
binding obligation of Genesee, enforceable against Genesee in accordance with
its terms, except as enforceability may be limited by bankruptcy, and by the
effect of general principles of equity (regardless of whether enforceability is
considered in a proceeding in equity or at law).

        (b) The execution, delivery and performance by Genesee of this Guaranty
are within its corporate powers, have been duly authorized by all necessary
corporate action, do not contravene or violate (i) its Certificate of
Incorporation or By-laws, (ii) any law, rule or regulation applicable to it, the
contravention or violation of which would or could be expected to have a
material adverse effect on Genesee's financial condition, (iii) any restrictions
under any material agreement, contract or instrument to which it is a party or
by which it or any of its property is bound, the contravention or violation of
which would or could reasonably be expected to have a material adverse effect on
Genesee's financial condition or (iv) any material order, writ, judgment, award,
injunction or decree binding on or affecting it or its property, the
contravention or violation of which would or could reasonably be expected to
have a material adverse effect or imposition of any adverse claim on the assets
of Genesee.

     7. Assignment.

        (a) Notwithstanding anything herein to the contrary, Genesee may assign
this Guaranty to its parent corporation or any affiliate thereof, provided that
the assignee assumes all of Genesee's obligations hereunder, including, but not
limited to, those set forth in Section 2 hereof, and provided that a copy of
such assignment and assumption is delivered to Boston Brewing. Upon such
delivery, Genesee shall be relieved of any obligations hereunder.

        (b) This Guaranty shall be for the sole benefit of Boston Brewing and
its parent corporation or any affiliate thereof and may not be assigned by
Boston Brewing other than as part of a corporate reorganization that does not
effect a change in control, within the meaning of Section 10(e) of the
Production Agreement, or enforced by anyone but Boston Brewing and its parent
corporation or any affiliate thereof, provided that no such assignment of this
Guaranty to the parent corporation or any affiliate of Boston Brewing shall be
valid unless such parent corporation or affiliate is also the assignee of the
Production Agreement.

     8. Entire Agreement; Amendments or Modification. This Guaranty together
with the Production Agreement (as modified hereby) and the Consent constitute
the entire agreement between the parties regarding the subject matter hereof,
and Genesee shall have no other liabilities or obligations of any kind to Boston
Brewing with respect to the Production Agreement or otherwise, except as
expressly provided herein. This Guaranty may not be amended or modified except
with a writing executed by both parties.
<PAGE>

     9. Governing Law. This Guaranty, as well as all rights and obligations of
the parties hereto, shall be given, construed and interpreted according to the
internal laws of the State of New York.

     IN WITNESS WHEREOF, Genesee has executed this Guaranty as of the date first
written above.

                                  THE GENESEE BREWING COMPANY, INC

                                  By:    MARK W. LEUNIG
                                         ---------------------------------------
                                  Name:  Mark W. Leunig
                                  Title: Vice President
Accepted and Agreed:
Boston Brewing Company, Inc. d/b/a
The Boston Beer Company, for itself
and as the sole general partner of Boston Beer
Company Limited Partnership

By:    RICHARD P. LINDSAY
       ---------------------------------------
Name:  Richard P. Lindsay
Title: Vice President