Massachusetts Mutual Life Insurance Company has agreed to buy First Mercantile Trust Company from SunTrust Banks Inc.

First Mercantile, Memphis, sells recordkeeping and investment management services to 3,100 retirement plans with a total of about 140,000 participants and $5 billion in assets under management, according to SunTrust, Atlanta.

MassMutual, Springfield, Mass., now manages about $40 billion in retirement plan assets.

MassMutual and SunTrust are not saying how much MassMutual is paying for First Mercantile, but they report that MassMutual plans to keep First Mercantile’s headquarters offices and the company’s current management team, including President Alan Dunaway and Chief Operating Officer James Pratt.

First Mercantile has about 125 employees. The focus on serving plans with about 50 to 250 employees and between $1 million to $20 million in plan assets.

First Mercantile also serves as trustee and fiduciary for about 180 collective investment trusts, according to SunTrust.

MassMutual likes First Mercantile’s collective trust business and its strong network of registered investment advisors, according to Elaine Sarsynski, the executive vice president who runs MassMutual’s retirement services division.