Taking one more small step toward regulating toxic polychlorinated biphenyls, or PCBs, a chemical whose manufacture was banned by Congress in 1976, the Environmental Protection Agency Friday set new controls on companies that produce the chemical as a byproduct.

The publication of the rule means there is one more shoe left to drop in the whole process of regulating the chemical as a byproduct, according to Bob Fensterheim of the Chemical Manufacturers Association. The chemicals, once widely used in electrical generators and transformers because of their high resistance to heat, have been branded as one of the most noxious pollutants because they are hard to break down and tend to become concentrated in fish and other foods, and in part because of liver and hormonal damage.

Friday's rule sets stringent controls on a few firms whose basic manufacturing process creates a high level of PCB byproducts; they will not be able to discharge amounts of the chemical that can be measured with present technology.

Paradoxically, however, chemical firms that produce moderate or low levels of PCBs in the factory will be able to discharge them in their wastewater. In practice, this means some firms will have to limit their discharges to one-tenth of a part per million of PCBs while others will be able to discharge up to 50 parts per million.

However, says CMA's Fensterheim, this discrepancy may be eliminated when EPA issues one remaining rule in the area.