C3H mouse embryo fibroblast cells, designated 10T1/2, can be transformed by physical and chemical agents including polycyclic aromatic hydrocarbons. In a previous report (Shen et al., Proc. Natl. Acad. Sci. USA 90, 11483-11487, 1993), we identified a cytochrome P450 gene induced by polycyclic aromatic hydrocarbons (PAHs) that is different from 1A1 or 1A2, and which we tentatively named P450CMEF. Here, we report the entire cDNA sequence of P450CMEF (5,128 bp) and the amino acid sequence deduced from it (543 residues). A comparison of the latter sequence with known cytochrome P450s indicates that P450CMEF is in a new subfamily of family 1 of the P450 superfamily. Accordingly, the Committee on Standardized Cytochrome P450 Nomenclature designated the gene Cyp1b1. Exposure to various aryl hydrocarbons (2.5 hr) induced Cyp1b1 mRNA in 10T1/2 cells to different degrees: 2,3,7,8-tetrachlorodibenzo-p-dioxin, 7,12-dimethylbenz[a]anthracene, benz[a]anthracene, benzo[a]pyrene, and beta-naphthoflavone were strong inducers; alpha-naphthoflavone and 3-methylcholanthrene, were moderate inducers; and benzo[e]pyrene was a weak inducer.