The Nobel Prize for Medicine was awarded yesterday to two American doctors who pioneered the use of organ and cell transplants to treat the severely ill.

Joseph E. Murray, 71, a kidney transplant specialist at Brigham and Women's Hospital in Boston, and E. Donnall Thomas, 70, a researcher at Seattle's Fred Hutchinson Cancer Center who perfected the use of bone marrow transplants to treat leukemia and other diseases, will share the $703,000 prize, Sweden's Karolinska Institute announced in Stockholm.

"Murray's and Thomas's discoveries are crucial for those tens of thousands of severely ill patients who either can be cured or given a decent life when other treatment methods are without success," the Institute said in its official citation.

"I am impressed by the way they had the courage to continue when other scientists said it was impossible," Professor Gosta Gahrton, a member of the Nobel committee, said at the news conference announcing the award.

Both men demonstrated how to dramatically reduce the possibility that the body's immune system will reject the transplant of an organ

or group of cells from another person.

Thomas's work involved bone marrow, the cells that inhabit the hollow center of many human bones and produce the white cells, red cells and platelets that are the major components of blood. In some forms of cancer, that cell-making machinery goes awry. In leukemia, for example, some of the body's white blood cells become cancerous and gradually crowd out the normal, blood-producing cells of the bone marrow.

Beginning in the 1950s, doctors began learning how to use chemotherapy to kill those cancerous cells. But they had no way of doing it without permanently wiping out all normal bone marrow cells and thereby killing the patient, which is why leukemia was such a lethal disease for so long.

Thomas pioneered a technique by which leukemia patients were given a new and healthy supply of bone marrow after all of their own blood cells were killed off by chemotherapy. The key to this procedure lay in closely matching the donor's tissue with the recipient's tissue, so that their immunological profiles would be as similar as possible and the chances of rejection as small as possible.

In more than 30 years of work supported by the National Cancer Institute, Thomas demonstrated that bone marrow transplants could be used successfully to treat a wide range of childhood and adult leukemias, as well as certain other cancers and a variety of non-malignant disorders affecting the bone marrow.

He began with identical twins, performing the first bone marrow transplant in 1956. Then, working with animals, he demonstrated that transplants also could be done successfully between individuals who were unrelated, provided that the immune systems of the donor and recipient could be carefully matched.

"In the 1950s, most people thought this would never succeed," said Thomas. "And in the 1960s, even people who had worked on it in the 1950s had given up. But we were convinced it could work."

"I'm very surprised to win the Nobel Prize," he said. "I really didn't think there was any chance that I would ever get it, because our work is so clinical. Most of the prizes recently have gone to basic scientists."

There is now an international registry of bone marrow donors which allows donors to be immunologically matched with seriously ill patients to minimize the chance that a patient will reject a transplant. Survival rates for some of the once-fatal diseases treatable by bone marrow transplants, such as chronic myelogenous leukemia and aplastic anemia, now approach 90 percent.

"There is always a need for someone who can straddle the gap between the laboratory and the clinic, and one of the great accomplishers of that was Don Thomas," said Gregory Curt, clinical director of the National Cancer Institute. "This award is not for a single achievement but for a whole series of achievements. . . . The Seattle people are real pioneers."

Murray, like Thomas, began his transplantation work in the 1950s. As a resident at Boston's Brigham Hospital at that time, Thomas cared for some of Murray's kidney transplant patients and the two men became friends. Murray said yesterday that his research also initially met with great skepticism from other scientists, who doubted that the body's immune system would accept a foreign organ.

But in 1954, Murray performed the first successful transplant of a human organ, in which a kidney was taken from one twin and given to another. Later, working in dogs, Murray showed that transplants were also possible in unrelated people if drugs were used to suppress the body's immunological reaction.

He also showed that kidneys could be taken from fresh cadavers and successfully transplanted into living patients, and that kidney transplantation was a reliable and effective treatment for renal disease.

"Murray did the original work, developed the operation as it is known today," said Folcert Belzer, chairman of surgery at the University of Wisconsin Hospital in Madison. "It was a completely new field. No one knew what would happen."

"He's a tough-minded but very collegial man," said Charles Carpenter, a colleague of Murray's at Brigham and Women's Hospital. "He is very open to discussion and ideas from all sources."

Murray retired from Brigham and Women's Hospital in 1986, and is now professor emeritus at Harvard Medical School.

The Nobel Prizes are the result of a bequest by the Swedish inventor of dynamite Alfred Nobel, who asked in his will that an award be given each year to those who have "conferred the greatest benefit on mankind."

This year marks the second consecutive year that Americans have swept the prizes for medicine. In all, Americans have won or shared 69 of the 142 medicine awards since 1901. Great Britain is second with 17 winners.