CHICAGO (AP) _ Federal racketeering charges were filed against 18 former employees of a now defunct commodities brokerage firm accused of defrauding 2,600 investors out of some $40 million.

The charges, announced Tuesday, are part of an on-going investigation of the nationwide operations of First Commodity Corp. of Boston.

U.S. Attorney Anton R. Valukas said $600 million in investor funds flowed through the company's offices across the country between the early 1980s and 1986, when the firm collapsed.

He said that in the largest investigation of its kind, federal investigators are probing the company's activities in Boston, New York, Miami and San Francisco, as well as Chicago.

Assistant U.S. Attorney Michele Smith said the probe is the largest in several aspects, including the number of victims - the 2,600 named in Tuesday's charges as well as the 44,400 other customers of the firm during the period of the alleged scheme.

''Allegations are that the entire company was operated under fraud,'' she said today.

The $600 million raised by First Commodity during the period from 1980 to the firm's collapse in February 1987 is alleged to have been fraudulently generated and is the largest sum involved in an alleged commodities fraud scheme, she said.

More than 500 FBI agents - the most ever in such a case - have been involved in the probe, Ms. Smith said. In November, three former salesmen for the company pleaded guilty in the investigation and agreed to cooperate with federal agents.

The defendants named Tuesday are charged with a total of 187 counts of racketeering, mail fraud, wire fraud and conspiracy.

They include David C. Connolly, 48, of Teaneck, N.J., the former national sales vice president for the company; Ross A. Barnard, 37, of suburban Hinsdale, former branch manager of the firm's Oak Brook office; and Stephen L. Barnard, 38, of suburban Flossmoor, former house broker of the Chicago office.

If convicted, they could face lengthy prison terms and thousands of dollars in fines.

Founded in the late 1970s, First Commodity Corp. grew rapidly into one of the nation's leading futures trading companies, but it collapsed beneath a mountain of investor lawsuits and federal investigations.

The former employees, named in a federal grand jury indictment and criminal information, are accused of using high-pressure sales tactics to persuade customers from around the country to invest in high-stakes commodity ventures misrepresented as being low-risk.

The defendants include the firm's former national vice president for sales and 15 former salesmen and managers from its former offices in Chicago and suburban Oak Brook, Valukas said.

In the alleged scheme, customers were solicited by telephone by First Commodity sales representatives posing as phony commodities experts. Those representatives often made repeated phone calls using ''scripts'' handed out by managers, Valukas said.

Investors were falsely told that ''commodity trading was not risky ... and would lead to high profits,'' he said.

''In fact ... the defendants knew almost all of FCCB's customers lost money,'' Valukas said.

During the alleged scheme, $100 million in sales was brought into the firm's Chicago and Oak Brook offices, and $63 million of that was lost, Valukas said.

More than $40.6 million of those sales and more than $22.7 million of the losses were attributable to the former employees named in the complaints, Valukas said. He declined to say whether more charges were forthcoming, but noted that the investigation is continuing nationwide.