An in vitro method for studying the interaction between tritiated benzo[a]pyrene ([3H]BP) and intact mouse skin was investigated. [3H]BP was applied to the epidermal surface of the skin floating on medium, and radioactivity in the medium was measured periodically. After 48-h incubation, about 45% of the radioactivity was in the medium; of this amount, about 80% accumulated during the first 24 h. Incubation temperature, metabolic inhibitors, and the amount of BP applied caused pronounced effects on accumulation; slight to moderate effects were noted for the type and volume of medium and vehicle used. Results of the temperature and inhibitor experiments indicated that accumulation of radioactivity depended on cell viability. About 80% of the radioactivity in the medium was water soluble, showing that biotransformation of BP by skin cells occurred. We are using the skin organ culture method to help understand the interaction of mammalian skin with hydrocarbon mixtures. A study of these interactions may lead to metabolic, cytotoxic, and macromolecular-binding profiles that characterize a carcinogenic mixture of hydrocarbons.