Exhibit 99.1350CERT

         Certification Pursuant to Section 906 of the Sarbanes Oxley Act

      I, Terry K. Glenn, President of Merrill Lynch Aggregate Bond Index Fund
      and Master Aggregate Bond Index Series (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: February 23, 2004


                                                /s/ Terry K. Glenn
                                                ------------------
                                                Terry K. Glenn,
                                                President of
                                                Merrill Lynch Aggregate Bond
                                                Index Fund and Master Aggregate
                                                Bond Index Series

      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 Aggregate Bond Index Fund and Master Aggregate Bond Index Series and
      will be retained by Merrill Lynch Aggregate Bond Index Fund and Master
      Aggregate Bond Index Series 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, Donald C. Burke, Chief Financial Officer of Merrill Lynch Aggregate
      Bond Index Fund and Master Aggregate Bond Index Series (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: February 23, 2004


                                                /s/ Donald C. Burke
                                                -------------------
                                                Donald C. Burke,
                                                Chief Financial Officer of
                                                Merrill Lynch Aggregate Bond
                                                Index Fund and Master Aggregate
                                                Bond Index Series

      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 Aggregate Bond Index Fund and Master Aggregate Bond Index Series and
      will be retained by Merrill Lynch Aggregate Bond Index Fund and Master
      Aggregate Bond Index Series and furnished to the Securities and Exchange
      Commission or its staff upon request.