The driver of the bus that crashed Sunday while speeding on a rain-slick bridge on I-70 outside Frederick, Md., fatally injuring six people, was taking medication related to a kidney ailment or condition, a federal investigator reported last night.

The investigator, Claude H. Harris, who is heading the probe for the National Transportation Safety Board, said it is not yet known whether the medication, which is under study and had not been precisely identified last night, could have affected the driving ability of driver George Whalan Brown, 68, or had a bearing on the crash.

Brown was killed in the wreck, and although an autopsy disclosed that he died of multiple injuries, it will be another week before results of chemical analyses will be available, Assistant Maryland Medical Examiner Dr. Dennis Smyth, who performed the autopsy, said yesterday.

Investigator Harris said Brown "was on some drug medication. There was a pillbox in his possession with several drugs at the time of the crash."

Brown, he said, had a history of kidney problems. Three months after going to work for the bus company, Baltimore Motor Coach, in June 1979, Brown took a leave in connection with those problems, and in 1982 had a kidney transplant, Harris said.

Brown returned to work as a regular driver for Baltimore Motor Coach last March, and had been medically certified to drive a bus by a physician in accordance with the rules of the federal Bureau of Motor Carrier Safety at that time.

The sixth person to die of injuries suffered in the Sunday crash was Roberto Harris, 73, of Baltimore, who died at 3:45 p.m. yesterday at the University of Maryland Shock Trauma Center in Baltimore. Eleven other persons were injured when the bus, on the way from Baltimore to the race track at Charles Town, W.Va., crashed.

Much of the investigation has focused on Brown, and both federal and state officials said yesterday there appears to have been error on his part. It was disclosed Monday that Brown had received 22 traffic tickets, including 13 speeding tickets, between 1956 and 1975 and that during that time his driver's license was suspended three times.

In another development yesterday, it was disclosed that the 1963 General Motors bus had a malfunctioning speedometer, but investigators rejected the speedometer problem as a contributing factor to the accident.

"We don't feel an inoperable speedometer is a defect that would have contributed to the accident," said Sgt. Bill Tower of the Maryland State Police. He said the speedometer problem was reported to the bus company by drivers who used the bus just before the Sunday crash.

"A person with driving experience would know within a few miles how fast they were going," Tower said, in explaining why the speedometer was considered "not that significant."

The bus was traveling 65 to 80 mph in the rain when it crashed, according to some witnesses, state police said.

Brown's driving record shows that his license was withdrawn for the first time in September 1956 after he received three tickets: one for speeding, one for reckless driving and one for failing to stop for an emergency vehicle, according to the Maryland Department of Motor Vehicles. He was fined $175 and his license was suspended for 60 days, records show.

Brown's other two suspensions occurred in 1964 after he accrued eight points for traffic tickets, according to the records. One suspension was for 15 days; the other for 30 days. Brown's driving record has been clean since 1975, DMV spokesman Steve Horwitz said.