Two Americans, Joseph E. Murray of Boston and E. Donnall Thomas of Seattle, were awarded the 1990 Nobel Prize in medicine Monday for discoveries that facilitated organ and cell transplants, including heart transplants.

Sweden's Karolinska Institute announced Monday it has jointly awarded the prize to Murray, 71, based at Brigham and Women's Hospital in Boston, and Thomas, 70, who works at the Fred Hutchinson Cancer Research Center in Seattle.The prize includes a cash payment of $700,000.

The institute said it honored the two for their discoveries concerning organ and cell transplantation "in the treatment of human disease."

"Joseph E. Murray discovered heart rejection following organ transplantation in man could be mastered, and E. Donnall Thomas managed to diminish the severe reaction that the graft can cause in the recipient - or the so-called graft versus host," the institute said.

The two men were the 65th and 66th Americans to receive the Nobel medicine prize since it was was first awarded in 1901. Of the 151 winners of the prize, 76 have been Europeans, 66 Americans, two Soviets and seven from other countries.

The institute said Murray, a retired Harvard professor, was the first to successfully transplant a kidney between homozygous twins - twins who have identical pairs of genes for any given pair of hereditary characteristics - and also pioneered kidney transplantations using kidneys obtained from people who have died.

"He pioneered transplantation of kidneys obtained from deceased persons and could show that patients with terminal renal insufficiency could be cured," the institute said. "The field was then open for transplantation of other organs such as liver, pancreas and heart.

The institute also said Thomas was successful in transplanting bone marrow from one individual to another and "showed that intravenously infused bone marrow cells were able to repopulate the bone marrow and produce new blood cells."

Such transplantations can cure severe inherited disorders such as thalassemia and disorders of the immune system, as well as leukemia and aplastic anemia.