The long-held suspicion that viruses may cause cancer in humans has strong new support because of two discoveries by scientists at the National Institutes of Health.

Members of one group report in today's issue of the journal Science that they have determined the exact, molecule-by-molecule structure of a cancer virus, a sarcoma virus, carried in the genes of mice.

At the same time, other NIH scientists have found sequences of the same family of viral genes, in this case viruses that cause leukemia, in human genetic material.

Together, these and other new discoveries add up to what many scientists call the best evidence yet that viruses are a cause of human cancers.

Some scientists in recent years have cast doubt on this, despite the known involvement of viruses in cancers of many animals, including apes, cows, cats, mice and chickens.

Then, just over a month ago, NIH's Dr. Robert Gallo reported isolating what he and his co-workers strongly believe to be a human leukemia virus. Japanese doctors report finding the same virus in some of their patients.

Days later, scientists at three centers--Cold Spring Harbor Laboratory on Long Island, Massachusetts Institute of Technology and Sidney Farber Cancer Institute in Boston--said they have found various "transforming genes" in human leukemia and colon, lung, breast and bladder cancer.

Viruses are strings of genetic material, like genes. In effect, they are genes that cause or help cause illness.

But the new, emerging concept of the role that viruses may play in human cancer makes these viruses much different from the simpler viruses that people "catch," like flu or measles or polio viruses.

In cancer, what scientists call the "viral genome," the genetic material of the virus, may be only one player in a still little understood cellular drama that leads to a cancer.

These viruses, or viral genes, may in some unknown circumstances interact with other genes to start making cancer cells in wild and uncontrolled numbers.

Though the whole drama is still little understood, new facts, and increased understanding of it, are emerging.

This is made possible by genetic engineering, the new "recombinant DNA" technology that has scientists chopping up DNA and RNA, the chemical materials of genes and viruses, and manipulating them.

"For the first time," Dr. E. Premkumar Reddy said yesterday, "we have the tools to manipulate genes. To totally sequence them decipher their structure , to understand their molecular organization and to use them to transform cells and see what happens.

"This is a new era. It is like the explosion of research in nuclear physics in the 1920s and '30s, research that totally changed the subject."

Reddy, Mary Jane Smith and Dr. Stuart Aaronson have deciphered the genome or genetic structure of the Moloney mouse sarcoma virus, one long known to cancer researchers.

The virus is a so-called Type C RNA retrovirus. The same kind of virus causes many known animal tumors and can be transmitted from parent to offspring in inherited genes.

It is Type C leukemia viral material that has been found in human DNA by a group led by NIH's Dr. Malcolm Martin. The Martin work was reported in the Proceedings of the National Academy of Sciences.

The mouse sarcoma virus, as described by Reddy, Smith and Aaronson is a string or chain of 5,828 "base pairs" or sets of individual nucleotides or basic chemicals whose exact order they now know.

The chain seems to have "hot spots" where it can combine with genes in cells to become the transforming genes that may lead to a cancer.

The separation technique that enabled Aaronson and his colleagues to identify the order of nucleotides was developed by Dr. Walter Gilbert of Harvard, for which Gilbert shared a Nobel prize.