Six people pleaded guilty to fraud yesterday in connection with a massive scheme to launder adverse credit ratings of hundreds of prospective buyers at virtually every car dealership in the area and at various department, furniture and home improvement stores.

The six - two former employees of the Credit Bureau, Inc., two car salesmen and two go-betweens - were the first to plead in the credit fraud scheme that prosecutors say may be among the most widespread frauds in Washington History. Prosecutors have said millions of dollars in credit may have been extended based on doctored credit histories.

The scheme has been under investigation by a grand jury for about six months, according to Assistant U.S. Attorney Roymond Banoun of the fraud division.

He said salesmen would tell customers with low credit ratings that they could have their credit histories improved by paying a fee of up to $800 each, the salesmen would then approach a middleman to have the credit history changed, and insiders at the Credit Bureau would actually carry out the doctoring of the computerized histories. "Everyone would receive a share of the money," Banoun said.

Entering guilty pleas yesterday were:

Roberta Vaughan, 27, of 2104 D St. SE, a supervisor of computer operators at the Credit Bureau, Inc., in Langley Park, Md. Vaughan pleaded guilty to doctoring the credit records of more than 300 persons over a two-year period at the director of someone outside the firm, and accepting up to $50 for each change she made.

Rosalyn Cleo Jordan, 30, of 1370 Oak St. NW, also an employee at the Credit Bureau, Inc., who said she was recruited by an outsider to alter credit histories last spring after Mrs. Vaughan was caught.

Robert (Bobo) Renfrow, 27, of 6003 Clay St. NE, a car salesman at Northeast Ford, who pleaded guilty to having his own credit rating improved by making illegal payments. Renfrow is a former boxer here whose last reported sports involvement were unsuccessful tryouts for a professional football team.

Carl Douglas Shaifer, 43. of 6810 District Heights Pkwy., District Heights, who was a car salesman at Seidel Chevrolet, Inc., and Sheehy Ford., Inc. Shaifer pleaded guilty to having a credit history altered for a man who later defaulted on the loan.

David Duane Martin, of 6808 Eldridge St., Landover Hills, who is the president of United Credit Association, a company that collects outstanding debts. Martin pleaded guilty to charges concerning his recruitment of "customers" who needed their credit ratings altered.

Delores Ruff Dorsey, 29, of 1907 14th St. NW, who contacted Jordan at the Credit Bureau as a possible replacement for Vaughan as the company "insider."

The six persons pleaded guilty to charges that included conspiracy, aiding and abetting, and making false statements on applications for bank loans.

Banoun said in court yesterday that the scheme would be begun when a customer's loan application would be rejected and returned, for example, to a car dealership. The salesman would tell the customer that he could arrange to have the customer's credit history altered for a fee, Banoun said.

At the same time, the salesman might suggest to the customer that he apply for a loan for a bigger car as long as his credit rating was going to be improved, investigators said. That way, the salesman would get a bigger commission from the car dealership.

The salesman would give a portion of the money to the middleman, who would then contact the "insider" at the Credit Bureau to actually doctor the files. The computer operator would erase any evidence of bad debt payments, bankruptcies, tax liens, or similar information and provide new data that would give the customer a much higher credit rating.

The go-betweens kept most of the money, investigations added.

Victims of the scheme included the lending institutions that relied on accurate credit information on which to base their loans, Banoun said.