Exhibit 11(a)

Sidley Austin LLP   SIDLEY AUSTIN LLP   | BEIJING    GENEVA       SAN FRANCISCO
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                    NEW YORK, NY  10019 | CHICAGO    LONDON       SINGAPORE
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                                         FOUNDED 1866

                                  June 14, 2006

Merrill Lynch Short-Term Portfolio, a series of
Merrill Lynch Municipal Bond Fund, Inc.
800 Scudders Mill Road
Plainsboro, New Jersey 08536

Ladies and Gentlemen:

      We have acted as counsel for Merrill Lynch Short-Term Portfolio (the "ML
Fund"), a series of Merrill Lynch Municipal Bond Fund, Inc. (the "ML Company"),
in connection with the proposed acquisition by the ML Fund of substantially all
of the assets, and the proposed assumption by the ML Fund of certain stated
liabilities, of BlackRock UltraShort Municipal Portfolio (the "BlackRock Fund"),
a portfolio of BlackRock FundsSM ("BlackRock Funds"), and the simultaneous
distribution to the BlackRock Fund of newly-issued shares of common stock of the
ML Fund having an aggregate net asset value equal to the assets of the BlackRock
Fund acquired by the ML Fund reduced by the amount of the liabilities of the
BlackRock Fund assumed by the ML Fund (collectively, the "Reorganization"). This
opinion is furnished in connection with the ML Company's Registration Statement
on Form N-14 under the Securities Act of 1933, as amended (the "Registration
Statement"), relating to shares of common stock, par value $0.10 per share, of
the ML Fund (the "Shares"), to be issued in the Reorganization.

      As counsel for the ML Company and the ML Fund in connection with the
Reorganization, we are familiar with the proceedings taken by the ML Company and
to be taken by the ML Company in connection with the authorization, issuance and
sale of the Shares. In addition, we have examined and are familiar with the
Charter and By-laws of the ML Company, each as amended and restated as of the
date hereof, and such other documents as we have deemed relevant to the matters
referred to in this opinion.

      Based upon the foregoing, we are of the opinion that subsequent to the
approval of the Agreement and Plan of Reorganization between the ML Company, on
behalf of the ML Fund, and BlackRock Funds, on behalf of the BlackRock Fund (the
"Agreement and Plan"), as set forth in the joint proxy statement and prospectus
constituting a part of the Registration Statement (the "Proxy Statement and
Prospectus"), the Shares, upon issuance in the manner referred to in the
Agreement and Plan, against payment of the consideration set forth in the
Agreement and Plan, will be legally issued, fully paid, and non-assessable
shares of common stock of the ML Fund.

      We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the use of our name in the Proxy Statement and
Prospectus constituting a part thereof.

                                                           Very truly yours,

                                                           /s/ Sidley Austin LLP

       Sidley Austin LLP is a limited liability partnership practicing in
               affiliation with other Sidley Austin partnerships