Two Americans and a Briton shared the Nobel Prize for Medicine on Monday for research that has led to new drugs used against a wide range of illnesses, including AIDS and heart disease.

Sweden's Karolinska Institute said Americans Gertrude Elion and George Hitchings and Briton Sir James Black laid the groundwork vital in developing medicines against leukemia, malaria, virus infections, gout and other maladies."For the moment I feel completely shocked," Black told reporters in London when informed he was sharing the $400,000 award. "The shock has almost overwhelmed the joy I feel."

Elion, reached at her home in Chapel Hill, N.C., said: "What do you think my reaction is? First of surprise, then disbelief, then elation." Hitchings said the award was "wonderful for my family."

The citation said that work in the late 1940s by Elion and Hitchings, of the Wellcome laboratories in North Carolina, plotted the differences between healthy human cells and cancerous organisms.

"Their original research could be utilized to develop drugs that selectively block the growth of cancer cells and of noxious organisms," the institute said.

The most recent application of their work was the development at Wellcome of azidothymidine (AZT), the most successful drug so far developed to treat the symptoms of AIDS, which kills by breaking down the body's immune system.

"They developed the first effective drug against a virus _ herpes," Professor Erling Norrby, of Karolinska's Nobel committee, told a news conference. "Their principles were a condition for the development of AZT," he said.

In 1977 Elion, 70, and Hitchings, 83, produced acyclovir, which inhibited the growth of the herpes virus.

Fellow researchers at Wellcome built on this breakthrough to produce AZT, which has proved effective in slowing the progress of the AIDS virus.

Black, 64, of King's College Hospital in London, pioneered the development of beta-blockers _ drugs used to ease the workload of the heart by reducing the oxygen supply, the institute said.

In 1962 his team developed the first clinically useful beta-blocker pronethalol, following it in 1964 with propranolol in 1964 which is used to treat angina and myocardial infarction.

"Black was the first to realize that the development of a clinically useful beta-receptor blocking drug might introduce a new pharmaco-therapeutic principle in the treatment of coronary heart disease," the institute said. It added that more recent research by Black had led to the development of a drug called cimetidine, which lay behind a new treatment for peptic ulcers.

Last year's medicine prize went to Japanese scientist Susumu Tonegawa, of the Massachusetts Institute of Technology, for research into the cure of a large number of auto-immune diseases and allergies.

Hitchings received a bachelor's degree from the University of Washington in 1927 and a doctorate from Harvard University in 1933.

After teaching at Harvard and Western Reserve University, he joined Burroughs Wellcome Co., a pharmaceutical firm, in 1942. Wellcome Research Laboratories is the company's research arm.

Elion earned her bachelor's degree from Hunter College and a master's from New York University. She has worked at Wellcome Research Labs since 1944 and as a senior research chemist since 1950. Since 1983, she has been a scientist emeritus at the laboratory and a research professor of pharmacology at Duke University.

This is the third of this year's six Nobel prizes to be awarded. The U.N. peacekeeping forces won the peace prize while the literature award went to 77-year-old Egyptian novelist Naguib Mahfouz.

The economics award will be announced by the Swedish Academy of Sciences on Tuesday, and the physics and chemistry prize winners will be named on Wednesday