A National Institutes of Health advisory committee in a secret session Monday conditionally approved one and turned down another request by corporations to field test two gene-engineered microbes previously confined to laboratories, committee members said yesterday.

Robert E. Mitchell, a California attorney who was acting chairman at the meeting of the Recombinant DNA Advisory Committee said that he feels it is important to keep the public informed about gene-engineering issues. He described the committee's actions in general terms. His descriptions were confirmed by other committee members.

The NIH has contended that the sessions on gene engineering must be closed to protect trade secrets of corporations voluntarily coming before the committee for approval of experiments in this highly competitive field.

Since the Monday meeting, NIH officials have refused to comment on the committee's recommendations, and have attempted to keep the advisory committee's votes from being made public.

Critics in this tremendously controversial field questioned why the final committee action could not be announced since that would not reveal secrets. NIH officials said that it was simply a matter of policy. But committee members said that they were not told about the policy or asked to vote to establish such a policy.

Gene-engineering experiments have become controversial in recent weeks after author Jeremy Rifkin, an opponent of gene engineering, filed suit against the NIH to halt the agency's approval of experiments that would release gene-engineered plants and microbes into the environment where, he fears, they might spread and possibly cause unforeseen damage. Three such experiments have been approved.

The committee Monday considered proposed experiments from Cetus Madison Corp. of Wisconsin and Biotechnica International of Cambridge, Mass.

Mitchell said the proposal from Cetus was approved "in principle," but Cetus was asked to come back with "clarifying" changes in the details of its proposal.

Cetus' director of research, Winston Brill, said that his company's experiment involves the field testing of plants whose genes have been altered to make them more resistant to an agricultural disease. He did not name the disease.

Brill said that the experiments with the plants have been successful in the lab and the greenhouse. And he said that he hopes to test the plants in the field in the next few months.

Mitchell said that the application from Biotechnica was voted down, at least for the present, because "a number of points were brought up against it, and they clearly had things to do before they could go into the open field."

He said that the Biotechnica experiment was not as ready to go into the field with a new organism as the well-tested Cetus experiment and other proposals that had come before the committee this year.

The Biotechnica experiments would add a genetically-altered strain of Rhizobium meliloti to the roots of alfalfa. The bacterium is one that coaxes alfalfa roots to grow nodules that make nitrogen fertilizer. The experiment will attempt to boost the fertilizing power of the bacterium, said Frank Cannon, senior scientist with the company.

The committee's recommendations now go to Richard M. Krause, director of the National Institute of Allergies and Infectious Diseases, for final action.