A third of the nation's controversial experiments to combine genes from unrelated forms of life would be exempted from federal safety rules under a proposed easing of the regulations early next year.

This estimate was made by government research officials as they began a public hearing yesterday on the changes - and simultaneously admitted ordering a Harvard scientist to suspend his gene research for apparent violation of their regulations.

The unprecedented order by the Nation Institutes of Health to Harvard Medical School and Dr. Charles A. Thomas Jr. was revealed in response to a Freedom of Information Act request by the Environmental Defense Fund.

The fund is one of several environmental groups which are calling for stricter, not more lenient, rules for genetic research. Yesterday Leslie Dach, EDF science associate, called it "shocking" that Thomas was apparently pursuing his research without NIH approval of his safety protocol, and NIH officials didn't know this until EDP called their attention to Thomas on Dec. 6.

Despite testimony by EDF and other environmental groups and serious reservations expressed by some scientists, most scientists agreed yesterday that the NIH safety guidelines for recombinant DNA research - work joining the DNA (deoxyribonucleic acid) or genetic material of different organisms - can be relaxed to some extent.

Dr. Bernard Talbott of the office of NIH's deputy director for science said there are currently some 300 recombinant DNA projects at universities and research centers. Of these, he estimated, about 100 would be exempted under the newly proposed NIH rules because they merely mix bacteria, virus, plant or animal DNA in ways many scientists think occur often in nature.

Under the proposed rules, such "normal" DNA mixing could proceed without restriction. Only abnormal mixtures would still fall under an elaborate safety code with some stricter clauses. The new rules could become final by next march or April.

But the Thomas case could provide new fuel for those who argue that even tighter rules are needed as more and more scientists and industries manipulate the genes that order all life.

The facts about Thomas experiment are still only partly known. He could not be reached for comment Dr. Donald Fredrickson, NIH director, would say only that NIH investigators visited Harvard last Monday. NIH yesterday wrote Harvard saying that Thomas' federally funded research "must not be carried out" now: and an NIH official will return to Harvard this Monday for further investigation.

Thomas resigned from Harvard last month, it was learned, and will leave soon to join the Scripps Research Foundation in San Diego Jan. 1. Last month the Harvard Crimson said he was leaving partly because of departmental politics, and partly because of a long-standing quarrel over his genetic research.

Another Harvard professor who asked not to be named said Thomas felt his laboratory was equipped to work at NIH's "P3" containment level for some of the riskiest experimnets, but Harvard's biohazards safety committee, a faculty group, felt the lab was safe only for less risky, "P2" work. As a result of this disagreement, a Harvard "memorandum of understanding and agreement" on his work was not received by NIH until Dec. 9, 1977, though it should have received and approved it in 1976.

The professor called Thomas "a brilliant, respectable" researcher. But EDF's Dach said a woman who worked in his laboratory told EDF that safety rules were often breached there.

Only last month Dr. William Rutter of the University of California and San Francisco embarrassed NIH by admitting another violation of its safety rules. Rutter said he inadvertently used an unapproved kind of DNA in an attempt to duplicate a rat insulin gene.

Rutter and colleagues destroyed their work to that date when they cound that the DNA had not yet been declared safe. But they did not tell NIH about this for many months ofr fear, Rutter said, of further "inflaming" the publci over fenetic research.

Fredrickson last month called the California scientists' decision to hide their mistake as "disturbing" and "inexcusable" event which should not happen again because of improved communication between NIH and the nation's universities.

The hearings on the proposed guidelines will continue at NIH today.

One area of of agreement between many of those who seek to ease the safety rules and those who would toughen them continued to emerge yesterday. Many persons on both sides think Congress should apply any safety tules to industry as well as to the college recipients of federal funds who are now covered by the NIH guideliness. Bills to do so bogged down in Congres this year.