Exhibit 99.1350CERT


         Certification Pursuant to Section 906 of the Sarbanes Oxley Act

     I, Robert C. Doll, Jr., Chief Executive Officer of Merrill Lynch Funds for
     Institutions Series and Master Institutional Money Market Trust (together,
     the "Fund"), certify that:

          1.   The N-CSR of the Fund (the "Report") fully complies with the
               requirements of Section 13(a) or 15(d) of the Securities Exchange
               Act of 1934; and

          2.   The information contained in the Report fairly presents, in all
               material respects, the financial condition and results of
               operations of the Fund.

     Dated: June 20, 2005

                                            /s/ Robert C. Doll, Jr.
                                            -----------------------
                                            Robert C. Doll, Jr.,
                                            Chief Executive Officer of
                                            Merrill Lynch Funds for Institutions
                                            Series and Master Institutional
                                            Money Market Trust

     A signed original of this written statement required by Section 906, or
     other document authenticating, acknowledging, or otherwise adopting the
     signature that appears in typed form within the electronic version of this
     written statement required by Section 906, has been provided to Merrill
     Lynch Funds for Institutions Series and Master Institutional Money Market
     Trust and will be retained by Merrill Lynch Funds for Institutions Series
     and Master Institutional Money Market Trust and furnished to the Securities
     and Exchange Commission or its staff upon request.



Exhibit 99.1350CERT


         Certification Pursuant to Section 906 of the Sarbanes Oxley Act

     I, William M. Breen, Chief Financial Officer of Merrill Lynch Funds for
     Institutions Series and Master Institutional Money Market Trust (together,
     the "Fund"), certify that:

          1.   The N-CSR of the Fund (the "Report") fully complies with the
               requirements of Section 13(a) or 15(d) of the Securities Exchange
               Act of 1934; and

          2.   The information contained in the Report fairly presents, in all
               material respects, the financial condition and results of
               operations of the Fund.

     Dated: June 20, 2005

                                            /s/ William M. Breen
                                            -----------------------
                                            William M. Breen,
                                            Chief Financial Officer of
                                            Merrill Lynch Funds for Institutions
                                            Series and Master Institutional
                                            Money Market Trust

     A signed original of this written statement required by Section 906, or
     other document authenticating, acknowledging, or otherwise adopting the
     signature that appears in typed form within the electronic version of this
     written statement required by Section 906, has been provided to Merrill
     Lynch Funds for Institutions Series and Master Institutional Money Market
     Trust and will be retained by Merrill Lynch Funds for Institutions Series
     and Master Institutional Money Market Trust and furnished to the Securities
     and Exchange Commission or its staff upon request.