Merrill Lynch & Co. is negotiating to buy controlling interest in Colquitt-Carruthers Inc. as the latest step in a plan by the nation's biggest stockbroker to set up a nationwide network of real estate agencies.

"We've had discussions with the company [Colquitt-Carruthers] which could lead to an agreement," said John F. Kelly, manager of public affairs for Merrill Lynch, the parent of Merrill Lynch Pierce Fenner & Smith.

Through Merrill Lynch Realty Associates the company already owns controlling interest in Chris Coile & Associates, which has 17 real estate offices in the Annapolis and Baltimore areas.

The negotiations with Colquitt-Carruthers came after months of efforts by Merrill Lynch to acquire one of the major Washington real estate brokers. b

Merrill is known to have approached Long & Foster Realtors and recently negotiated to buy Mount Vernon Realty.

Long & Foster handles the Washington business of Merrill Lynch Relocation Service, a subsidiary of the brokerage firm which helps major companies arrange transfers of employes.

Since it set up the relocation service almost five years ago, Merrill Lynch has moved aggressively into other parts of the real estate business, with an announced goal of acquiring affiliates in the 15 largest metropolitan areas in the nation, Kelly said.

So far, Merrill has picked up seven real estate firms with more than 50 offices. In most cases, Kelly said, Merrill Lynch has paid cash for a majority interest in the firm, keeping the old owner as a minority partner to run the business.

Details of the Colquitt-Carruthers deal have not been disclosed.