Since the establishment of the more tumor susceptible SENCAR stock of mice, it has been used extensively in carcinogenesis assays by government agencies, academia, and commercial laboratories. The model has facilitated substantial cancer research involving mechanistic studies, as well as testing of various classes of chemicals, and complex mixtures such as organic extracts of drinking water, automobile exhaust, and consumer products, all of which are of particular concern to the regulatory agencies. The purpose of the EPA Workshop on the SENCAR Mouse in Toxicological Testing is to provide a forum for the discussion of research data related to the use of the animal model. The workshop critically addresses the performance of the SENCAR mosue as a carcinogenesis assay model.