A government board has approved the first release into the environment of a gene-engineered microbe--one designed to retard the formation of frost on plants--but four organizations are expected to file suit today to halt the experiment.

The gene is in a bacterium that is found in fields and on plants and normally helps start frost crystals when the temperature falls below 32 degrees. A National Institutes of Health board has approved field tests of an altered version that does not produce frost even when temperatures fall as low as 23. Since most frost damage occurs between 25 and 30 degrees, the new bacterium has enormous commercial potential, researchers say.

But the four organizations contend in their court suit that the experiment could be hazardous. In the worst case, opponents speculate that the frost-preventing bacterium might rise into the upper atmosphere and disrupt the natural formation of ice crystals there, which could affect the global climate.

They also claim that the NIH board is unqualified to assess the environmental risk of such an experiment since it has no ecologists or botanists, and has failed to carry out an environmental assessment of the experiment's dangers.

Such environmental assessments or impact statements are required by the National Environmental Protection Act on "major federal actions." The suit's plaintiffs, which include the Humane Society of the United States, say approval of the research constitutes "major" action.

The NIH offered no comment on the suit yesterday, but scientists familiar with the experiment said it would be hard to argue that it was risky, since very similar experiments with frost-stopping bacteria already have been carried out successfully with no apparent risk. Those experiments altered the bacterium in similar ways, but by methods other than gene-engineering and so were beyond NIH regulation.

The experiment--to be carried out by Steven Lindow of the University of California at Berkeley and partly funded by Advanced Genetic Sciences company of Berkeley--was approved by the National Institutes of Health under federal rules governing gene engineering.

Until research was delayed recently, Lindow had planned to spray a field of potatoes this month with an altered version of the extremely common organism called pseudomonas syringae. The natural organism lives on the outside of plants, and makes a chemical that encourages the formation of frost and ice a degree or so below freezing.

Lindow and his colleagues have identified the genes in the organism that trigger this quick-freezing action, and have excised them from the bacterium.

These neutered organisms are sprayed on the plants as soon as they sprout. Once the bacteria are in place, they take over the niche usually occupied by their frost-causing relations, and so crowd out any of the frost-causing variety before they can take hold.

The altered bacteria protect plants down to about 23 degrees, and possibly lower. Department of Agriculture officials said successful results could save millions of dollars in crops.

Besides the Humane Society, plaintiffs in the suit are the Environmental Task Force Inc., Environmental Action Inc. and the Foundation on Economic Trends. Leading them is Jeremy Rifkin, author of "Algeny," a book on genes, ecology and the future of man.

In the suit, expected to be filed in U.S. District Court here, the plaintiffs said introducing gene-engineered bacteria into the environment is much like introducing a foreign plant into the country.

"Some of our most significant problems are of that nature," suit documents said. "These include the kudzu weed, the chestnut blight, the gypsy moth, Dutch elm disease, and starlings and house sparrows, both of which are agricultural pests."

The suit was bolstered by affidavits from several scientists, including Eugene P. Odum of the University of Georgia and and David Pimentel of Cornell University, two of the country's most eminent ecologists. Odum and Pimentel did not criticize the Lindow experiment, but said they were worried about releasing newly engineered organisms into the environment without first testing to see their effects.

NIH defenders say the frost-stopping bacteria already exists on virtually all plants. The experiments do not add new genes or new powers to the bacterium, but simply take away some of its destructive powers.

The NIH committee also has approved two other experiments in which gene-engineered organisms will be released into the environment, but those experiments--involving altered varieties of corn, tomatoes and tobacco--were not scheduled for field tests this fall.