V79 Chinese hamster lung cells expressed low but significant aryl hydrocarbon hydroxylase activities when treated with an inducer of cytochrome P-450I, such as benz[a]anthracene or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), together with aminophylline. Inducibility by polycyclic aromatic hydrocarbons and inhibition by a specific monoclonal antibody indicated that the observed enzyme activity was mediated by cytochrome P-450I. Intact V79 cells pretreated with TCDD and aminophylline for 24 h metabolized benzo[a]pyrene to phenolic products with accumulated linearly in the growth medium for at least the same time period. Exposure of V79 cells to 10 μM benzo[a]pyrene and aminophylline for 72 h reduced subsequent cell growth by about 40%. The results demonstrate that V79 cells, under specific conditions, express PAH-inducible cytochrome P-450I and are capable of activating benzo[a]pyrene to cytotoxic products.