Defense Department plans to build a new germ-warfare laboratory at the Dugway Proving Grounds in Utah should be stopped, an environmental activist group argued last week in U.S. District Court here, because dangerous microbes might escape to threaten civilian populations.

The Army, which has run Dugway as a chemical and biological warfare testing station for more than 40 years, told the court the new lab is only a modernization intended to improve protection against the escape of microbes already being studied under less stringent conditions.

The activist group, on the other hand, alleged that the Army wants the lab to experiment with new and potentially deadlier microbes than have been used in the past. Such research could be aimed at improving defenses against possible new Soviet germ weapons, the group said, but the same research also could be used to develop offensive weapons, a step prohibited under a 1972 treaty outlawing development of biological weapons. The new lab would give Dugway, the source of a 1968 nerve gas leak that killed 6,000 sheep on nearby ranches, a facility using the most stringent safety standards devised by the government for containment of potentially lethal microorganisms.

The case is being watched closely by scientists, among whom there is widespread opposition to biological warfare. Several prominent biologists, including Nobel Prize winners, have questioned the purpose of the proposed lab.

The hearing, before U.S. District Court Judge Joyce Hens Green, was on a suit filed by the Foundation on Economic Trends, a private group led by Jeremy Rifkin, a critic of genetic engineering research. Rifkin's suit, asking for a permanent injunction against construction of the new lab, was filed last November, shortly after the Pentagon's plans came to light almost by accident.

The Pentagon had tried to fund the new lab through a routine request to House and Senate appropriations committees seeking reallocation of money that had already been authorized for other purposes. In a list of minor projects that the Pentagon wanted, such as parking garages and housing, was a $1.4 million "Aerosol Toxin Test Facility" at Dugway. (Aerosols, droplets small enough to be carried by the wind, are considered a prime means of delivering biological weapons.)

To reallocate the money, the Pentagon needed approval from the chairmen and ranking minority members of both subcommittees on military construction. All signed off, but then Sen. Jim Sasser (D-Tenn.), a ranking minority member, realized that the Dugway lab was not necessarily a routine item.

Sasser withdrew his approval, arguing in a letter to Defense Secretary Caspar W. Weinberger that the lab could give the Army a treaty-violating ability to test offensive biological weapons and should be submitted to the full Congress as a new program.

In response, Weinberger assured Sasser that the United States would not develop or possess biological or toxin weapons, although he said there was evidence that the Soviets were continuing to develop them. Weinberger said the lab was needed to test U.S. defenses such as detectors and filters against "known or suspected" Soviet biological weapons.

Shortly after Sasser's action, Rifkin and his group filed suit on grounds that the Pentagon had not prepared an environmental impact statement of the sort required if the Defense Department were going to embark on a new program. No matter how secure the new lab, he contended, accidents can happen and dangerous organisms could escape.

Gary Randall, a Justice Department lawyer representing the Defense Department, based his defense on the contention that no such statement was necessary because the new lab entailed no change in Dugway's activities. It was, he said, merely an improvement in environmental protection measures in an existing building for programs that have been under way for years.

Rifkin's lawyer, Edward Lee Rogers, contended that Weinberger's emphasis on the need to meet a new Soviet threat in biological warfare shows that the lab would be using new microbial weapons.

The hearing lasted one day.