Industries are being faced with the possibility of meeting discharge limits for toxicity as States begin to enforce the water quality standards prohibiting the discharge of toxic materials in toxic amounts. These effluent limitations create the need for an effective toxicity monitoring tool. The City of Baltimore, with assistance from the U.S. EPA, is in the process of evaluating monitoring tools to characterize the toxicity of industrial contributors to the Patapsco Wastewater Treatment Plant (PWWTP). The paper presents the results of biological toxicity reduction tests performed on five major industrial contributors to the PWWTP to evaluate the effectiveness of various monitoring tools as indicators of toxicity to the PWWTP biomass and potential toxicity pass-through. Toxicity monitoring tests investigated were Microtox, ATP, chemical fractionation, COD removal, and specific oxygen uptake rate (SOUR).