Plymouth (Mass.) County Retirement System issued an RFP for a general consultant, said John F. McLellan, chairman and treasurer of the $475 million plan. Regulations call for contracts to be put out to bid every five years or so, and incumbent Wainwright Investment Counsel is welcome to bid, he said. Interested parties should contact William Farmer, plan administrator, by March 11. System officials will evaluate the responses but will not make a decision until the end of April at the earliest, Mr. Farmer said.