Transformation into permanent cell lines of hamster embryo fibroblasts was induced by benzo(a)pyrene and 7,12-dimethylbenz(a)anthracene. Cells of the transformed lines had altered karyotypes, and the benzo(a)pyrene-induced line was tumorigenic in hamsters. Several lines of mouse fibroblasts of altered morphology were obtained following pyrene and benzo(a)pyrene exposure, but these cells were not tumorigenic when injected in cortisonized mice. 7,12-Dimethylbenz(a)anthracene inhibited the cellular proliferation of an untransformed line (3T3) of mouse fibroblasts to a lesser extent than in a polyoma virus-transformed variant of the same line. At doses that were not toxic to growth of mass cultures, exposure to 7,12-dimethylbenz(a)anthracene caused a reduction in the subsequent cloning efficiency of the 3T3 line. Similar inhibitory effects were obtained with tobacco smoke condensate, a phorbol ester fraction from Croton tiglium L. and benzo(a)pyrene. The toxic effect of bcnzo(a)pyrene was demonstrable 20 generations after exposure. No transformation was observed in the 3T3 cell line with any chemical treatment. The toxic and transforming effects of carcinogens in cell culture systems is discussed.