The driver of the bus that crashed Sunday outside Frederick, Md., killing six people, was certified as physically fit June 19 even though the insulin prescribed for his diabetes should have disqualified him from driving the bus, officials said yesterday.

The federal Bureau of Motor Carrier Safety regulations specifically forbid a person taking insulin from interstate commercial driving because of the possibility of a blackout, officials said.

"The concern is that there can be a lack of control . . . a loss of consciousness" among persons required to take insulin, said Gerald Davis, chief of the agency's regulations division. The driver's doctor said, however, that he had no history of such problems.

Driver error has been cited as the cause of the accident, which occurred when a Baltimore Motor Coach Co. bus that some witnesses estimate was traveling 65 to 80 mph, hit a guardrail on a rain-slick I-70 bridge. The other 11 people on board were injured.

Among the six killed was the driver, George Whalan Brown, 68, who died of multiple injuries. Brown had been taking insulin injections twice a day for the past six months to control his blood sugar, one of his doctors said yesterday. Dr. Matthew Weir, one of the medical directors of the kidney transplantation service at University of Maryland Hospital in Baltimore, said that seven drugs, including insulin, had been prescribed for Brown, who had his second kidney transplant on Feb. 26.

Weir said that he would have certified Brown as physically able for some jobs, but not for bus driving "because the chance always exists of blackout" for a person who needs insulin to control his blood sugar. Weir said that Brown's health was generally good, and his recovery from the transplant had gone "extremely well" despite his age. He said Brown checked his own blood sugar at home "regularly with his glucose test machine.

"He was very well controlled, but the chance always exists for a blackout," Weir added.

Claude H. Harris, head of the investigation for the National Transportation Safety Board who spent the day combing Brown's employment file with Baltimore Motor Coach, said Brown obtained his June 19 medical certification from Dr. Pedro DeBorja, a private physician in Baltimore. Harris said that DeBorja told him that he had tested Brown for diabetes but that the test was negative.

Harris also said that DeBorja told him that Brown provided reports from his other doctors showing "that he was physically able to work."

DeBorja declined to discuss the case yesterday.

Harris said that laboratory tests completed so far show that there was no evidence of any alcohol in Brown's system. But it will be several more days before tests will reveal whether Brown's medical problems or any of the drugs that he was taking contributed to the accident, Harris said.

Those drugs, according to Dr. Weir, included three antirejection medicines for the kidney transplant, a diuretic for hypertension, an antibiotic for a urinary infection, sodium bicarbonate because of the kidney transplant, and insulin.

Weir said Brown needed the insulin to control the diabetes that developed after he began taking antirejection drugs for the kidney transplant.

Brown's first kidney transplant was performed Sept. 26, 1982, Weir said. But seven months later, in April 1983, Brown's system rejected the kidney and it had to be removed, the doctor said. When he stopped taking the antirejection drugs, he stopped taking insulin, Weir said.

With the second kidney transplant this year, the antirejection drugs were resumed and so was the insulin, Weir said.

Harris said that Brown worked as a commercial driver of buses and trucks from about 1957 to 1979 when his kidney problems worsened.

He went to work for Baltimore Motor Coach Co. in 1979 but left after four months because of his kidney problems, Harris said.

"To the best of our knowledge, he didn't operate a commercial vehicle between 1979 and 1985," which would include the time he was having kidney problems and was first on insulin, Harris said. Brown went back to work for Baltimore Motor Coach after his June 19 certification.