Hundreds of new gene-joining experiments, called potentially dangereous by many scientists only a few years ago, were freed from almost all restrictions yesterday.

Accepting the current view of most biologists that most such experiments will not cause harm. Dr. Donald Fredrickson, director of the National Institutes of Health, issued new federal safety rules that, in effect, ease or remove most of the existing rules.

The action is expected to double and redouble quickly the nearly 800 such experiments under way today in universities and research centers. It is expected to pave the way for a vast industrial effort to use the new gene engineering to make insulin, brain hormones, disease-fighting chemicals and, probably, new products such as improved food crops.

In the new work, genes from man, animals or bacteria are isolated or chemically reconstructed, then made to grow and reproduce in colonies of bacteria. A fear stated at first by many scientists, and today by far fewer, is that some of the new genes or changed bacteria might get out of the laboratories and cause epidemics.

Dr. Bernard Talbot, special assistant to Frederickson, estimated that in the next year scores of new laboratories will join the 130 now active. And he forecast "a huge increase" in the current 780 registered experiments to separate and recombine DNA, the genetic material that is common to man, animals, viruses and bacteria.

In addition, Talbot said, two firms attempting to make drugs are already combining partly synthesized human genes or sections of them with bacteria in batches of hundreds of liters.

NIH rules governing federally funded tests will continue to limit experiments to 10 liters (2.6 gallons) of any batch of genes growing in bacteria -- commonly, modified versions of the intestinal bacteria, E. coli. It is in bacteria that the new genes grow and "express themselves."

NIH has given Eli Lilly, the Indian-apolis drug-making giant, permission to produce two parts of the insulin gene in 150-liter (39-gallon) batches of E. coli. Lilly wants to make a purer, more plentiful insulin than the animal insulin diabetics now use.

Genentech, a small but aggressive, sophisticated new San Francisco firm, has been given permission to try to make two human hormones -- insulin and the growth-regulating brain hormone, somatostatin -- in 750-liter (195-gallon) batches.

These hormones and, most recently, the body's bacteria-fighting substance, interferon, have all been made in small laboratory amounts by isolating or reproducing the genes that govern their manufacture, then transplanting the genes to E. coli. The E. coli then become hormone-making factories.

The NIHguidelines legally apply only to research centers getting federal funds. The Carter administration has urged Congress to require that industry also comply with the rules, but no bill ever got out of committee.

However, several companies, Lilly and Genentech included, have voluntarily agreed to abide by the federal rules. NIH officials said they know of no firm attempting such work that has not agreed.

One of the new procedures promulgated yesterday is a way for firms to register their research plans voluntarily with NIH. NIH in turn promises to keep the plans secret from commercial competitors.

To minimize dangers, the first 1976 NIH guidelines said each experiment must be done in one of four types of laboratories, depending on risk. Almost any hospital bacteriological laboratory where personnel take precautions to avoid infection could qualify for the least restrictive standard. The strictest was the kind of sealed lab found at the old Army germ war laboratories.

The new rules downgrade the requirements for 80 percent of all current recombination-DNA research. And "about half of the 80 percent," said Talbot, may now be done in the simplest laboratory.

These simplest experiments using modified E. coli need no longer even be reported to NIH, through they must be screened by a local safety committee. All the changes, said Talbot, will make this work "cheaper and easier" -- and more common.