The EPA and the City of Baltimore (City) entered into a cooperative agreement to perform a TRE research study at the City's Patapsco Waste Water Treatment Plant (Patapsco WWTP). The Patapsco WWTP TRE represents one of the first case histories of a toxics management program at a municipal wastewater treatment plant. The study characterized treatment plant influent and operations data and related these data to wastewater toxicity and inhibition data; characterized the plant's influent and effluent wastewater toxicity using definitive EPA bioassays and in-plant toxicity monitors; evaluated the fate of toxicity and specific toxic chemicals during treatment; and evaluated techniques to trace toxicity to its source and to identify specific components of toxicity. A major reduction in toxicity was achieved by the Patapsco WWTP, substantial acute and chronic toxicity was present in the Patapsco WWTP effluent. The wastewater fractionation/identification procedures identified nonpolar organic compounds as the principal toxicants while compounds removed by volatilization and ammonia contributed lesser amounts of toxicity.