The $310 million Middlesex County Retirement System, East Cambridge, Mass., hired Mellon Bank as its custodian, replacing BayBanks. The change was made because the fund wanted a bank with strong international capabilities, said Jack Walsh, executive director. The California Bankers Association, San Francisco, hired First Interstate Bank to provide a bundled 401(k) plan service program for member banks. Nine diversified mutual fund investment options will be offered in the program a combination of First Interstate's proprietary funds and funds from other money managers, said Christopher Chenowith, a senior vice president at the association. First Interstate will provide daily valued record keeping, trust and administration services, employee investment education and communication. Two of the 24 banks using the association's program converted to the new First Interstate program Oct. 1. The rest of the plans, including the association's own employee 401(k) plan, will convert to the new program Jan. 1. The association's old program provides four investment options, none of which will be retained. The previous record keeper was Administrative Systems.