The metabolism of benzo[a]pyrene (BP) to water-soluble derivatives was measured in cell cultures derived from poikilothermic and homeothermic vertebrates. Tritiated BP was added to the cultures and, after 24 hours of incubation, the medium was extracted with chloroform:methanol:water. The amount of BP metabolized per cell (relative efficiency of metabolism) was calculated from the 3H-BP equivalents in the aqueous phase of the extraction mixture. Many poikilothermic vertebrate cell cultures, including those derived from fish, toad, lizard, and turtle, were able to metabolize BP. In some instances, metabolism was as efficient as in rodent cell cultures. Cell multiplication was inhibited by BP, only in those cultures in which the metabolism of BP was highly efficient. Conversely, cytotoxicity was not observed in cell cultures with little or no ability to metabolize the carcinogen.