A Washington physician and a former Washington attorney well known in boxing circles entered guilty pleas in U.S. court here this week in a scheme involving up to $600,000 in false and inflated medical insurance claims.

The attorney, Harry I. Barnett, who now lives in Ocean Ridge, Fla., entered his guilty plea late yesterday afternoon before U.S. District Judge Barrington D. Parker to three counts of mail fraud.

Dr. Marshall D. Nickerson Jr., of 77 G St., NW, entered his guilty plea before Parker on Tuesday on one count of mail fraud and one count of illegally prescribing narcotics.

The pleas are part of a massive federal investigation into alleged insurance fraud comitted by several Washington doctors and lawyers, according to Assistant U.S. Attorney C. Madison Brewer of the major crimes division.

Police officials said the insurance probe involves several million dollars' worth of claims, and that the related narcotics investigation of Nickerson involved prescriptions worth $9 million.

Barett, a sometime fight promoter and currently a consultant to boxers in the Flordia area, during the three-year scheme reportedly drove a Rolls-Rocye with personalized license plates and took several trips to a house he is believed to own in the Virgin Islands.

According to statements in court and an affidavit in support of a search warrant for Barnett's law offices here at 1226 17th St. NW, the investigation began last year after one of Barnett's clients filed a complaint with the consumer fraud section of the D.C. Police Department.

The clients told police that Barnett had shown him various documents in connection with settlement of an auto accident claim. One of the documents was a bill for treatment by Dr. Nickerson, whom the client said he never had seen.

As the investigation developed, officials said, they discovered that Barnett often had filed various phony documents in support of claims for his clients and had them return one-third of their award plus medical fees to him.

When he lived in the Washington area, Barnett also had offices at 1108 16th St. NW and resided at 2901 Plainview Dr., Potomac.

Nickerson's drug charge involved his writing of prescriptions for Dilaudid," a pain-killer used by addicts to heighten the effects of heroin. He is believed to have written more than 2,000 prescriptions for a total of 200,000 tablets of the drug, with a street value police estimated by police at $9 million.

The two men will be sentenced on Jan. 28.