Exhibit 99.1350CERT

         Certification Pursuant to Section 906 of the Sarbanes Oxley Act

I, Robert C. Doll, Jr., Chief Executive Officer of Aggregate Bond Index Fund of
Merrill Lynch Index Funds, Inc. and Master Aggregate Bond Index Series of
Quantitative Master Series 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: February 24, 2005

                                          /s/ Robert C. Doll, Jr.
                                          --------------------------------------
                                          Robert C. Doll, Jr.,
                                          Chief Executive Officer of
                                          Aggregate Bond Index Fund of Merrill
                                          Lynch Index Funds, Inc. and Master
                                          Aggregate Bond Index Series of
                                          Quantitative Master Series 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 Aggregate Bond Index Fund of
Merrill Lynch Index Funds, Inc. and Master Aggregate Bond Index Series of
Quantitative Master Series Trust and will be retained by Aggregate Bond Index
Fund of Merrill Lynch Index Funds, Inc. and Master Aggregate Bond Index Series
of Quantitative Master Series 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, Donald C. Burke, Chief Financial Officer of Aggregate Bond Index Fund of
Merrill Lynch Index Funds, Inc. and Master Aggregate Bond Index Series of
Quantitative Master Series 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: February 24, 2005

                                          /s/ Donald C. Burke
                                          --------------------------------------
                                          Donald C. Burke,
                                          Chief Financial Officer of
                                          Aggregate Bond Index Fund of Merrill
                                          Lynch Index Funds, Inc. and Master
                                          Aggregate Bond Index Series of
                                          Quantitative Master Series 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 Aggregate Bond Index Fund of
Merrill Lynch Index Funds, Inc. and Master Aggregate Bond Index Series of
Quantitative Master Series Trust and will be retained by Aggregate Bond Index
Fund of Merrill Lynch Index Funds, Inc. and Master Aggregate Bond Index Series
of Quantitative Master Series Trust and furnished to the Securities and Exchange
Commission or its staff upon request.