WASHINGTON (AP) _ American laboratories are examining blood samples from Soviet citizens as part of a joint U.S.-Soviet study of the long-term health effects of the Chernobyl nuclear accident, Dr. Robert Gale said Thursday.

Gale, the first American doctor called in to help after the nuclear accident last April, said the study will guarantee top medical care to the 135,000 people who lived near the plant and provide further scientific knowledge about radiation-caused cancer and genetic mutations.

''Our obligation as physicians transcends political differences,'' Gale said in a speech at the National Press Club. ''We're helping ourselves by helping the Soviets. We have twice as many reactors. We're obligated to learn as much as we can.''

Gale, a medical professor at the University of California at Los Angeles and chairman of the International Bone Marrow Transplant Registry, said genetic abnormalities are showing up in blood samples from Soviet citizens and plant workers.

He said the incidence likely would increase because radioactive cesium will be present in the Soviet environment for 300 years.

''Cancers are the most immediate threat,'' Gale said. But he noted that mutations could pose a more serious long-term problem because they can be passed on to other generations.

Asked about reports that Soviet workers entombing the contaminated plant are being exposed to radiation levels far higher than would be permitted here, Gale said that is unlikely.

''We are not in the Soviet Union as inspectors. We are there as physicians,'' said Gale, who spends two or three weeks a month in Moscow. ''But I believe they have been very careful in monitoring the dose of radiation that people are allowed to receive.''

Gale also said high levels would be detected in the blood samples under study in U.S. labs.

The doctor said he believes Soviet data on the level of radiation in crops and food probably is ''fairly reliable ... My tendency is at this point to believe them. They've been very frank.''

He said the Soviets have tended to overestimate the consequences of the Chernobyl accident, particularly in the projected number of excess cancers it will cause. U.S. experts believe Soviet estimates - which postulate a minimum of 10,000 excess cancers and 5,000 deaths - may be 10 times too high, he said.

Of almost 500 people hospitalized after the accident, Gale said, 31 died and nine are still in the hospital. The rest remain at home.

He said patients exposed to high levels of radiation were saved through the use of sophisticated sterile environments; a new antibiotic used to fight the herpes virus; reinjection of blood cells drawn from the victim right after the accident, and then frozen, and bone marrow transplants.

Thirteen patients received the transplants and four survived, said Gale, who added that was the expected survival rate. He said patients who received transplants of fetal livers, which contain what he called ''immunologically naive bone marrow,'' died of burns so the treatment could not be evaluated.

In other comments, Gale said:

-The radiation released in the Chernobyl accident was roughly equivalent to 10 percent of the voluntary releases in atmospheric testing during the 1950s and 1960s.

-Risks to travelers are small. You would get a higher radiation dose flying to Kiev than by staying there for a year, eating the food and breathing the air.