Petitioner John Hancock Mutual Life Insurance Company (Hancock) and respondent Harris Trust and Savings Bank (Harris), the current trustee of a corporations retirement plan, are party to Group Annuity Contract No. 50 (GAC 50), an agreement of a type known as a "participating group annuity." Under such a contract, the insurer commingles with its general corporate assets deposits received to secure retiree benefits, and does not immediately apply those deposits to the purchase of annuities. During the life of the contract, however, amounts credited to the deposit account may be converted into a stream of guaranteed benefits for individual retirees. Funds in excess of those that have been so converted are referred to as "free funds." Dissatisfied over its inability to gain access to GAC 50s free funds, Harris filed this suit pursuant to, inter alia, the Employee Retirement Income Security Act of 1974 (ERISA), alleging that Hancock is managing "plan assets," and therefore is subject to ERISAs fiduciary standards in its adm