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                                                                     EXHIBIT 8.1
                                                                     S-4
                               Quarles & Brady LLP
                            411 East Wisconsin Avenue
                         Milwaukee, Wisconsin 53202-4497


                                 August 28, 2000



First Northern Capital Corp.                 Mutual Savings Bank
201 North Monroe Avenue                      510 East Wisconsin Avenue
Green Bay, WI  54301                         Milwaukee, WI  53202

Bank Mutual Corporation
4949 West Brown Deer Road
Brown Deer, WI  53223


Ladies and Gentlemen:

         We have acted as counsel to Mutual Savings Bank, a Wisconsin mutual
savings bank ("Mutual Savings"), Bank Mutual Corporation, a federally-chartered
corporation ("Bank Mutual") and Mutual Savings Bancorp, MHC, a
federally-chartered mutual holding company (the "MHC"), in connection with (i)
the Merger, as defined and described in the Agreement and Plan of Merger, dated
as of February 21, 2000, by and among Mutual Savings, OV Corp., a Wisconsin
corporation and a newly formed, wholly-owned subsidiary of Mutual Savings, and
First Northern Capital Corp., a Wisconsin corporation ("First Northern") (the
"Merger Agreement"), and (ii) the preparation and filing of the Proxy Statement
of First Northern and Prospectus of Bank Mutual relating to the Merger (the
"Proxy Statement/Prospectus"). With First Northern's consent, OV Corp. will have
assigned all of its rights under the Merger Agreement to Bank Mutual, and Bank
Mutual will have assumed all of the obligations of OV Corp. under the Merger
Agreement. Unless otherwise indicated, each capitalized term used herein has the
meaning ascribed to it in the Merger Agreement.

         In connection with this opinion, we have examined the Merger Agreement,
the Proxy Statement/Prospectus and representations contained in letters to us,
dated on or about the date hereof, by Mutual Savings (on behalf of itself, Bank
Mutual and the MHC), Bank Mutual and First Northern. For purposes of this
opinion, we have assumed (i) the accuracy of the representations made by Mutual
Savings, Bank Mutual and First Northern contained in their respective letters to
us, and (ii) that the Merger will be consummated in the manner described in the
Merger Agreement and the Proxy Statement/ Prospectus.

         Subject to the assumptions set forth above, we are of the opinion that
(i) the Merger will be treated, for federal income tax purposes, as a
reorganization within the meaning of Section 368(a) of the Internal Revenue Code
of 1986, as amended (the "Code"), and that First Northern and Bank Mutual will
each be a party to the reorganization, within the meaning of Section 368(b) of
the Code, and (ii) the statements made under the captions "Summary -- Summary of
the Merger -- Tax Consequences; Tax Opinion" and "The Merger and The Merger
Agreement -- Federal Income Tax Consequences" in the Proxy Statement/Prospectus,
to the extent they constitute matters of law or legal conclusions, are correct
in all material respects.

         The Merger Agreement provides that the shareholders of First Northern
will receive shares of common stock of Bank Mutual equal to the "Stock
Percentage." The Stock Percentage will be selected by Mutual and will be



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a percentage somewhere between 40% and 70% of the total consideration received
by First Northern shareholders. Although the Internal Revenue Service ordinarily
does not give rulings on merger transactions, its ruling policy would require
that at least 50% of the consideration to be paid in the Merger be in the form
of Bank Mutual stock for the tax consequences as described above to be realized.
However, such ruling policy is not necessarily the Internal Revenue Service's
interpretation of the existing law, as there are court decisions which have
upheld tax free reorganization treatment where less than 40% of the
consideration was in the form of stock.

         This opinion does not address any federal, state, local or foreign tax
consequences that may result from the Merger or any other related transactions,
other than as set forth above. Further, our opinion is not binding upon the
Internal Revenue Service or the courts, and there is no assurance that the
Internal Revenue Service could not successfully assert a contrary opinion.

         In rendering our opinion, we have considered the applicable provisions
of the Code, Treasury Department regulations promulgated thereunder, pertinent
judicial authorities, interpretive rulings of the Internal Revenue Service and
such other authorities as we have considered relevant. It should be noted that
statutes, regulations, judicial decisions and administrative interpretations are
subject to change at any time (possibly with retroactive effect). A change in
the authorities or the accuracy or completeness of any of the information,
documents, corporate records, covenants, statements, representations or
assumptions on which our opinion is based could affect our conclusions. This
opinion is expressed as of the date hereof, and we are under no obligation to
supplement or revise our opinion to reflect any changes (including changes that
have retroactive effect) (i) in applicable law or (ii) in any information,
document, corporate record, covenant, statement, representation or assumption
stated herein which becomes untrue or incorrect.

         This opinion is intended solely for the benefit of Mutual Savings, Bank
Mutual, Bank Mutual's shareholders, First Northern and First Northern's
shareholders in connection with the Merger. It may not be relied upon for any
other purpose or by any other person or entity, and it may not be made available
to any other person or entity without our prior written consent.

         We consent to the filing of this opinion as an exhibit to the Proxy
Statement/Prospectus and the reference to our firm under the caption "The Merger
and The Merger Agreement -- Federal Income Tax Consequences; Tax Opinion" in the
Proxy Statement/Prospectus. In giving such consent, we do not admit that we are
"experts" within the meaning of Section 11 of the Securities Act of 1933, as
amended, or that we are within the category of persons whose consent is required
by Section 7 of the Securities Act or the rules and regulations of the
Securities and Exchange Commission thereunder.
                                                 Very truly yours,

                                                 /s/

                                                 QUARLES & BRADY LLP