The U.S. Environmental Protection Agency has been given the responsibility for regulating the release of toxic chemicals into the environment. Accordingly, the EPA presently conducts an extensive and comprehensive research program to determine the adverse effects of environmental factors on human health. A considerable amount of this research activity is directed toward toxicological testing and test development. The Health Effects Research Laboratory of the EPA, in Cincinnati, Ohio, has specifically focussed on the task of determining, through the use of appropriate short-term assays, the potential health hazards of the complex mixtures of organic compounds found in drinking and waste waters.