After more than nine months of negotiations, the chemical industry and two leading environmental groups have agreed on a way to regulate low-level concentrations of polychlorinated biphenyls (PCBs).

Their remaining task is to get the Environmental Protection Agency to agree to go along.

The unusual joint proposal was submitted to the EPA last week by the Chemical Manufacturers Association, the Environmental Defense Fund and the Natural Resources Defense Council, traditional adversaries whose legal representatives meet more frequently in the courtroom than across a negotiating table.

At issue are small quantities of cancer-causing PCBs that sometimes are byproducts when other chemicals are manufactured. Last October, the EPA proposed to study and regulate each of more than 100 manufacturing processes that might produce PCBs.

The prospect horrified the chemical industry, which envisioned years of uncertainty while the agency plodded through its studies, proposed rules and sorted out court challenges. The environmentalists were no less aghast, envisioning a battalion of EPA regulators tied up with low-level PCBs to the exclusion of other, potentially more hazardous materials.

"We realized we seemed to have a joint interest in resolving this in such a way that wasn't resource-intensive," said Chemical Manufacturers Association official Bob Fensterheim.

Jacqueline M. Warren, counsel for the environmental groups, said the joint proposal was a "reasonable way to deal with what is actually a small problem. It's a positive step, but then it was not a problem of the magnitude of the use of PCBs in electrical equipment."

PCBs that are generated accidentally amount to no more than 100,000 pounds a year, according to Warren, compared with the 700 million pounds of highly concentrated PCBs that are still in use, primarily as a conducting agent in electrical equipment.

In addition, some of the PCBs created during chemical processes break down faster when released into the environment and thus pose less of a long-term health hazard. The more chlorine atoms that are attached to the compound, the more hazardous it is; many of the byproduct PCBs have only one or two chlorine atoms.

Under the joint proposal, chemical manufacturers would be required to assure that PCB concentrations in their products average less than 25 parts per million and never go higher than 50 parts per million.

Concentrations released into the air would be restricted to no more than 10 parts per million, and those released into water would be restricted to a tenth of a part per million or less.

The joint proposal took into account the lesser hazards posed by the mono- or dichlorinated biphenyls, allowing the manufacturer to "discount" them when calculating concentrations of PCBs. For example, 50 parts of monochlorinated biphenyls would be considered the equivalent of one part, when determining the concentration of PCBs.

An EPA spokesman this week called the proposal a "welcome" step and said that, while it might not be accepted, it will get "high priority" treatment.

Last month, the EPA was able to settle a long and nettlesome quarrel between environmentalists and the steel industry with the help of a negotiated agreement on water pollution standards.

Weeks earlier, the Chemical Manufacturers Association had joined forces with the Environmental Defense Fund in a lawsuit against the government, both seeking the establishment of a congressionally mandated agency that would study the health effects of hazardous substances.

While such alliances are far from common, Fensterheim suggested that the PCBs proposal demonstrates that at least they are not impossible.

"There aren't too many like this," he said. "But hopefully there will be a greater sense of cooperation between industry and such groups as EDF. Everybody has an interest in avoiding litigation."