Effluents from wastewater treatment facilities are discharged as a complex mixture of numerous chemical substances, which may include cytotoxic, carcinogenic and mutagenic compounds. The U.S. Environmental Protection Agency has recently established a research program to determine if a bioassay approach for evaluating the potential adverse human health effects from exposure to complex mixtures might supplement conventional chemical analysis for setting regulatory limits for wastewaters. The paper summarizes a bioassay testing strategy for characterizing cytotoxic and mutagenic activity of various wastewater effluents. The use of a relative potency framework for assessing complex mixtures for potential health hazards is addressed.