In the process of in vitro cell transformation, normal cells, which have an oriented pattern of growth and a limited life span in vitro and which are not tumorigenic, are converted into cells that have a hereditary random pattern of growth, the ability to grow continuously in culture, and the ability to form tumors. Such heritable phenotypic changes may arise from alterations in gene expression due to somatic mutations after interaction of the carcinogen with due to somatic mutations after interaction of the carcinogen with cellular DNA. Our studies have indeed shown (a) that metabolically activated carcinogenic polycyclic hydrocarbons which have been shown to bind to cellular DNA induce somatic mutations in mammalian cells; (b) that there is a relationship between the degree of mutant induction and the degree of carcinogenicity of the different hydrocarbons tested; and (c) that the somatic mutations were induced by metabolites rather than by the hydrocarbons themselves. In the case of benzo(a)pyrene (BP), a very common carcinogenic polycyclic hydrocarbon, its 7,8-diol-9,10-oxide was identified as the major mutagenic and cell-transforming metabolite. Based on these studied, it was possible to estimate the genetic target size for cell transformation by comparing in the same cells the frequency of cell transformation and mutation for ouabain resistance (which is presumably due to a mutation at one locus) induced BP and by one of its major metabolites. The results indicated that the target size for transformation is 20 times larger than that determined for ouabain resistance. This suggests that cell transformation, as determined by a hereditary pattern of cell growth, may be due to a mutation and that this mutation can occur in one out of a small number of the same or different genes.