Exhibit 99.1350CERT

         Certification Pursuant to Section 906 of the Sarbanes Oxley Act

I, Terry K. Glenn, President of Master Enhanced International Series (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: August 21, 2003


                                            /s/ Terry K. Glenn
                                            ------------------------------------
                                            Terry K. Glenn,
                                            President of
                                            Master Enhanced International 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 Master Enhanced International
Series and will be retained by Master Enhanced International 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 Master Enhanced International
Series (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: August 21, 2003


                                            /s/ Donald C. Burke
                                            ------------------------------------
                                            Donald C. Burke,
                                            Chief Financial Officer of
                                            Master Enhanced International 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 Master Enhanced International
Series and will be retained by Master Enhanced International Series and
furnished to the Securities and Exchange Commission or its staff upon request.