Aryl hydrocarbon (benzo(a)pyrene) hydroxylase is present and inducible in Buffalo rat liver cells in culture. There is substantial variation in both basal and inducible hydroxylase activities among heteroploid subclones isolated from a heteroploid parent population, and among diploid subclones isolated from a diploid parent population. This variation is not related to differences in the growth characteristics of the subclones, or to differences in their chromosome number. The results indicate that substantial heterogeneity in both basal and induced hydroxylase activity develops during the growth of both heteroploid and diploid cell strains in culture. These findings indicate that diploid cell populations are not necessarily homogeneous with respect to aryl hydrocarbon hydroxylas activity. This observation may complicate the interpretation of experiments involving somatic cell hybridization or polycyclic hydrocarbon-induced transformation and/or cytotoxicity. This heterogeneity in hydroxylase activity develops rather rapidly (2-3 mo of culture), in the absence of any apparent mutational stress.