A human cytochrome P-450 (P450) 1B1 cDNA was expressed in Saccharomyces cerevisiae and the microsomes containing P450 1B1 were used to examine the selectivity of this enzyme in the activation of a variety of environmental carcinogens and mutagens in Salmonella typhimurium TA1535/pSK1002 or NM2009 tester strains, using the SOS response as an end point of DNA damage. We also determined and compared these activities of P450 1B1 with those catalyzed by recombinant human P450s 1A1 and 1A2, which were purified from membranes of Escherichia coli. The carcinogenic chemicals tested included 27 polycyclic aromatic hydrocarbons and their dihydrodiol derivatives, 17 heterocyclic and aryl amines and aminoazo dyes, three mycotoxins, two nitroaromatic hydrocarbons, N-nitrosodimethylamine, vinyl carbamate, and acrylonitrile. Among the three P450 enzymes examined here, P450 lB1 was found to have the highest catalytic activities for the activation of 11,12-dihydroxy-11,12-dihydrodibenzo[a,l]pyrene, 1,2-dihydroxy-1,2-dihydro-5-methylchrysene, (+)-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene, 11,12-dihydroxy-11,12-dihydrobenzo[g]chrysene, 3,4-dihydroxy-3,4-dihydrobenzo[c]phenanthrene, 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole, 2-aminoanthracene, 3-methoxy-4-aminoazobenzene, and 2-nitropyrene. P450 1B1 also catalyzed the activation of 2-amino-3,5-dimethylimidazo[4,5-f]quinoline, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, 2-amino-3-methylimidazo[4,5-f]quinoline, 2-aminofluorene, 6-aminochrysene and its 1,2-dihydrodiol, (-)-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene, 1,2-dihydroxy-1,2-dihydrochrysene, 1,2-dihydroxy-1,2-dihydro-5,6-dimethylchrysene, 2,3-dihydroxy-2,3-dihydrofluoranthene, 3,4-dihydroxy-3,4-dihydro-7,12-dimethylbenz[a]anthracene, and 6-nitrochrysene to appreciable extents. However, P450 1B1 did not produce genotoxic products from benzo[a]pyrene, trans- 3,4-dihydroxy-3,4-dihydrobenzo[a]anthracene, trans-8,9-dihydroxy-8,9-dihydrobenzo[a]anthracene, 7,12-dimethylbenz[a]anthracene and its cis-5,6-dihydrodiol, 5-methylchrysene, 11,12-dihydroxy-11,12-dihydro-3-methylcholanthrene, 1,2-dihydroxy-1,2-dihydro-6-methylchrysene, benzo[c]phenanthrene, 2-amino-6-methyldipyridol[1,2-a:3',2'-d]imidazole, 2-acetylaminofluorene, benzidine, 2-naphthylamine, aflatoxin B1, aflatoxin G1, sterigmatocystin, N-nitrosodimethylamine, vinyl carbamate, or acrylonitrile in this assay system. P450 1B1 is expressed constitutively in extrahepatic organs, including fetal tissue samples, and is highly inducible in various organs by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds in experimental animal models. Thus, activation of procarcinogens by P450 lB1 may contribute to human tumors of extrahepatic origin.