An American doctor here to help treat victims of the Chernobyl nuclear power station accident said today that the death toll, now nine, is certain to rise.

Dr. Robert Gale, of the University of California Medical Center in Los Angeles, said at a press conference today that the casualty figures "may change substantially."

Gale, a leading expert on bone marrow transplants, arrived here May 2, six days after the explosion at the power plant 80 miles north of Kiev, and was joined within days by two colleagues from the University of California and one from Israel.

Since their arrival, they have been closeted at a Moscow hospital, working with a team of Soviet doctors to save the lives of 35 victims suffering from acute radiation.

Gale said today that 19 bone marrow operations had been performed so far and that the other patients either do not need the operations or are suffering from irreversible damage to other organs. So far, seven radiation patients have died, in addition to two men killed by steam and falling debris during the accident.

Dr. Richard Champlin, also of the UCLA Medical Center, said 33 of those suffering acute radiation poisoning are men and two are women. He said six of the 19 victims who had bone marrow transplants have died.

"We were predictably not successful in all cases," said Gale today, "but 28 are still alive and, although we know that additional deaths are unavoidable, we hope most will survive."

In a transplant, bone marrow from a close relative, or someone else with very similar tissue structure, is injected into a patient's veins. It then replaces the marrow destroyed by radiation.

Gale appeared at the press conference for Soviet and foreign journalists with Dr. Andrei Vorobev, who is head of the Soviet team handling the Chernobyl patients. All of the 28 now in serious condition are hospitalized in Moscow. In all, a total of 299 are in hospitals here and in other cities being treated for lesser degrees of radiation.

The American doctor said brothers, sisters and parents of the victims had donated bone marrow.

Also present at today's press conference was Armand Hammer, the American industrialist who has spent a lifetime promoting U.S.-Soviet trade and who subsidized the four doctors' trips, as well as supplies of medicine and equipment.

Gale and Hammer were invited to the Kremlin today for a meeting with Communist Party leader Mikhail Gorbachev, who greeted them warmly in the presence of a flock of Soviet and American reporters.

"I am very happy to meet you; I talked about you on television last night," said Gorbachev to Gale. The Soviet leader thanked Gale and his colleague from UCLA, Dr. Paul Terasaki, in a televised speech last night about the Chernobyl accident. Israeli specialist Yair Reisner is also assisting in the treatment of the radiation victims.

To Hammer, Gorbachev expressed the hope that the current level of U.S.-Soviet cultural exchanges would increase the "first flow" to a "real river."

"Recent days have given a lot of food for thought," he noted.

At today's press conference, Hammer said the Soviet government had offered to reimburse him for his medical donations but that he had refused.

Both Gale and Vorobev emphasized that the number of patients from Chernobyl will fluctuate, given the range of exposure seen among people who were near the accident.

In general, irreversible cases are those involving exposure to more than 800 rads of radiation, Gale said. But serious consequences can be found among people with less exposure, depending on what organs were most exposed, the doctors explained today.

The unpredictability of the radiation cases from Chernobyl is compounded by radioactive gases that apparently were released at the height of the accident, the doctors said.

Gale today complimented his Soviet colleagues for their early efforts during the crisis, saying that they were "extremely well prepared and made as good an estimate as possible under the circumstances."

Gale is head of the advisory committee for the International Bone Marrow Transplant Registry, a group involving 60 countries that has made plans to treat victims of radiation poisoning in case of a nuclear emergency. Before Chernobyl, the Soviet Union was not a member of the organization, but it is now applying for membership.

Given the uniqueness of the Chernobyl accident, Gale said no one could have been fully prepared. Still, he said, the medical community would have to learn lessons from the incident, because "anything less than 100 percent efficiency is unacceptable."

Vorobev said most of the people in serious condition had been on duty at the power plant on April 26 when an explosion occurred at 1:23 a.m.

"There were some people who were not at the station but nearby," he said, noting that it was difficult for some patients to relate exactly where they were and how they were exposed to radiation. He said that in one case, a man had high radiation levels on one side of his body, but not the other.

"We try to reproduce where he stood during the accident but it is a very complicated job, especially considering the split-second releases of uncontrolled radioactive gases," he said.

Vorobev said it would be "unrealistic" to expect to find any cases of acute radiation poisoning outside the 18-mile zone that has been sealed off around the Chernobyl reactor.

Gale noted that people farther away are "extremely unlikely" to suffer severe radiation damage, but, he said, "they may suffer long-term consequences."

Gale declined to predict the long-term medical prospects for people living in the vicinity of the power plant.

Vorobev said today that among the inhabitants of Pripyat, the town closest to the Chernobyl reactor, "there are no persons who have" radiation poisoning. He said he had examined people from Pripyat and found levels of 50 rads, or one-half the minimum danger level.

Vorobev said today there were no health risks in Kiev, but Gale refrained from characterizing the hazards, noting that he had not been to the region to make measurements himself.

"I don't believe we should be making definitive statements about radiation at points distant the Chernobyl source," he said. "I would very much welcome an opportunity to go down there."