The Environmental Protection Agency approved another experiment yesterday intended to release genetically engineered microbes into farm fields, even as lawsuits and protests continue to beset such tests.

The agency agreed to permit two California plant pathologists to test whether a genetically altered bacterium can protect young potatoes from frost. The work is to be done near Tulelake by Steven E. Lindow and Nikolas Panopoulos of the University of California at Berkeley.

Theirs is one of four experiments approved or near approval that will release genetically engineered bacteria or viruses into fields or farm animals.

All have encountered trouble, including local government action, citizen protest and court action by activist groups.

The Monsanto Co., for instance, may be blocked at least temporarily from attempting to put a live, genetically engineered bacterium on corn roots to protect against pests.

The EPA is expected to rule within a week on Monsanto's field tests in Charles County, Mo., but the county commission has said that the test plot is in a flood plain and that the experiment would violate county ordinances.

Monsanto also faced opposition from the Agriculture Department, which said one strain of bacteria the company wanted to test may become a plant pest itself, necessitating more tests.

The EPA suspended a permit issued to Advanced Genetic Sciences Co. (AGS) of Oakland to experiment with an altered bacterium because the company was found to have tested it before receiving permission and made deliberately false statements in its application.

The Tulelake experiment could begin within two weeks, if local officials do not block it. Supervisors in Siskiyou and Modoc counties are to discuss the matter within a week.

A group calling itself Concerned Citizens of Tulelake has gathered about 450 signatures on a petition seeking to delay the experiment, according to Djuanna Anderson, a leader of the group. She said about 1,300 people live in the area.

The biotechnology industry also faces another problem as its first field tests begin. Insurance companies have expressed reluctance to write liability policies covering the possibility of severe environmental damage by a live engineered microbe.

In many cases, insurors are unwilling to issue such policies, and in others the cost is prohibitive, according to Jeffrey Gibbs, assistant general counsel for the Association of Biotechnology Companies (ABC). He said one company was offered a $1 million policy for a $400,000 annual premium.

"A lot of companies are going bare," operating without coverage, he said.

About 15 companies are working through the ABC to establish their own insurance company outside the United States, probably in the Caribbean region.

The Tulelake experiment is being challenged by activist Jeremy Rifkin and his Foundation on Economic Trends, which said the University of California system will lose liability insurance covering the experiment as of July 1.

Rodney Umscheid, the university system's director of risk management and safety, confirmed that yesterday. He said the system then will self-insure for the first $1 million and buy policies to cover further damages up to perhaps $100 million.

The Tulelake experiment is very similar to the suspended Oakland test because pathologist Lindow helped develop both.

In both experiments -- AGS's on strawberries and Lindow's on potatoes -- the object is to prevent frost formation. Plant leaves usually have colonies of a bacterium, Pseudomonas syringae, that can trigger early frost formation.

Lindow uses genetic-engineering techniques to cut out the piece of the genes that makes a chemical that acts as a starter-crystal in frost formation.

He plans to bathe potato-seed pieces with the remaining bacteria, in hopes of colonizing the budding potato so that it has no room for natural bacteria containing the frost-triggering chemical.