A policy to develop water-quality-based permit limitations for toxic pollutants entering treatment plants is being developed because, even with efficient removal of conventional pollutants, significant amounts of toxic substances are being discharged through the Nation's wastewater treatment systems. To support the policy of water-quality-based permit limitations on toxic pollutants, an overall water-quality-based toxic control process is evolving. A key step in this overall control process is a Toxicity Reduction Evaluation (TR E) at an individual municipal or industrial wastewater treatment plant. After a toxics water quality problem is confirmed at a plant, the TRE (which involves an integrated monitoring approach employing both specific chemical and bioassay techniques) (1) evaluates the toxicity reduction occurring across the plant and determines the probable cause of the excessive toxicity in the discharge (2) traces the toxicity to its source and, at the same time, identifies the components, if possible, and (3) evaluates control alternative to eliminate the excessive pass-through. Some data from a toxicity reduction survey of six Ohio municipal wastewater treatment plants are given. The progress on toxicity reduction research suggests that the TRE concept may be an affordable approach to solve the site-specific problems of toxicity discharges by treatment plants.