Much effort has been placed on the development of short-term assay systems that use the induction of mutations in bacterial and mammalian cells as the basis for predicting the carcinogenicity of environmental chemicals. However, evidence has accumulated that suggests that most environmental chemicals require enzymatic activation to produce metabolites that will react with cellular macromolecules, and many or the bacterial and mammalian cell lines identified are unable to produce that activation. Consequently, scientists began to use microsomal systems to supply activation. In this study, two short-term assays were examined and cells that appeared capable of metabolizing various carcinogens were identified. In the first method, the metabolism of tritiated benzo(a)pyrene (B(a)P) to aqueous-acetone soluble products was measured. While this method is useful to identify cells that are capable of metabolizing B(a)P or related compounds, it is not applicable to other classes of compounds. Therefore, a second, more general method was examined. This second method detects the formation of agents that damage DNA by measuring DNA synthesis inhibition after exposure to a carcinogen.