The Soviet Union is prepared to let American doctors follow up in the treatment and observation of thousands of people affected by the Chernobyl nuclear accident, according to a U.S. physician involved in the case.

Dr. Robert Gale, of the University of California, said in an interview that Soviet officials have said they are willing to allow the U.S. National Academy of Sciences and other research organizations to participate in surveys kof the approximately 100,000 people living in the Chernobyl region and potentially affected by radiation.

The medical information collected from the sample would be invaluable in determining the causes, possibly the cures, for cancer, Gale said.

"We have to follow these cases. We owe it to these people to give them the best possible care," he said.

"It would also have a tremendous impact on our understanding of cancer and tell us a lot about its causes."

Gale said he had been given the go-ahead to solicit American participation by Deputy Health Minister Oleg Shohepin and, in principle, by Soviet leader Mikhail Gorbachev during a meeting earlier this month.

"I never met with any resistance," Gale said.

Gale, a bone marrow transplant specialist who is back in the Soviet Union for a second visit to follow up on the care of patients, said U.S. doctors and researchers could help in the massive health survey, with both computerization and patient care.

"The longest term from Chernobyl will be in the scientific evidence about cancer," he said. "If international science misses this opportunity, we have lost a lot.

During his first two-week stay here, Gale and three other western doctors -- including one from Israel -- helped perform a total of 19 bone marrow transplants on patients from Chernobyl severely injured by radiation from the April 26 nuclear power plant explosion.

The most recent official death toll from the accident was put at 19 -- of whom 17 had died in hospitals.

In Cologne, West Germany, today, in a closed session of the sixth Congress of International Physicians for the Prevention of Nuclear War, the organization that won the Nobel Peace Prize last year, Soviet doctors put the death toll at 21 and said that of the 19 bone marrow transplant patients, 11 had died.

Soviet doctors have said that six of the 19 transplants had already taken place before Gale arrived in Moscow on May 2.

Gale said that if half of the transplant patients lived, "I think we would be very lucky."

Gale declined to give out new casualty figures, which he said change daily. "People are dying every day," he said. But he added that he is pressing Soviet officials for clarification of numbers that have been become increasingly confusing.

In Cologne, Soviets put the number of injured who were transferred to Moscow from Chernobyl within two days after the accident at 1,000 although the initial number hospitalized was said to be about 100.

In interviews with the Soviet press published yesterday and today, Dr. Angelina Guskova, described by Gale as head of the Soviet emergency care team, said that specialists from Moscow had been dispatched to the Chernobyl region four hours after the disaster.

Within 24 hours, she said, the 100 "most serious" cases out of 1,000 had been selected and brought back to Moscow.

On May 14, Gorbachev put the total number hospitalized at 299 - the number that Gale said he believes is still the base figure of people treated for radiation, including those who have died.

He said he thought the number of serious cases - put at 35, including the fatalities - had not changed, even though GUskova was quated as saying 80 patients were in 'extreme condition." He said he thought that number included those still hospitalized.

The confusion-compounded by articles in the Soviet press giving varying accounts of the victims' heroism, of the initial treatments, and even of the hospitals - has been disconcerting, and showed a problem with central coordination, Gale said.

"They've got too many people talking about it. Everybody and their uncle is talking about it,' he noted in an interview tonight. "Perhaps it is all in an effort to be frank, but it is very confusing."

"They may have overdone it in terms of trying to give access to data, on both the 35 seriously affected and others who may have radiation illness, will be to refine techniques to determine the dosage of radiation.

In the cast of the Chernobyl patients, he said, the medical history was lacking to give a quick and accurate reading on dosages.

The best method, he said, was through conversations later - learning how soon after exposure the patients vomited, or experienced nausea. Often it is much later, sometimes as much as one or two weeks, before signs of radtation burns appear on the skin, he said.

Washington Post staff writer Dusko Doder added from Cologne:

Dr. Yevgeny Chazov, a Soviet cardiologist and cochairman of International Physicians for the Prevention of Nuclear War, said that the accident at Chernobyl had required a tremendouss mobilization fo medical resources - with a ratio of three medical workers per casualty.

This, he said, showed the medical profession and facilities would be helpless to deal with the casualties in a nuclear war, even if only a few nuclear bombs were detonated.