Using the liver 9000 x g supernatant fraction of uninduced rats and monospecific antibodies against microsomal reduced nicotinamide adenine dinucleotide phosphate-cytochrome P450 reductase and two inducible forms of cytochrome P-450, PB-P-450 (major cytochrome P-450 component of the liver microsomes of phenobarbital-treated rats) and MC-P-448 (major cytochrome P-450 component of the liver microsomes of 3methylcholanthrene-treated rats), in the Ames test system, the contributions of these 2 forms of cytochrome P-450 to the mutagenicities of 3-amino-l-methyl-5H-pyrido[4,3-b]indole, 2acetylaminofluorene, and aflatoxin B1 were studied. The mutagenicities of these three carcinogens were completely inhibited by antibody to reduced nicotinamide adenine dinucleotide phosphate-cytochrome P-450 reductase. The mutagenicities of 3-amino-l-methyl-5H-pyrido[4,3-b]indole, 2-acetylaminofluorene, and aflatoxin B1 were inhibited 85, 85, and 40%, respectively, by antibody to MC-P-448 (anti-MC-P-448 immunoglobulin) and also inhibited 5, 30, and 60%, respectively, by antibody to PB-P-450 (anti-PB-P-450 immunoglobulin). These results indicate the importance of these two forms of cytochrome P-450 in the activation of the carcinogens by the liver 9000 x g supernatant fraction of uninduced rats. We also examined the correlation between the induction of the two forms of cytochrome P-450 and the change of mutagenic activities at various time points after a single dose of various carcinogens to rats. Benzo(a)pyrene and 3-methylcholanthrene induced only MC-P-448 about 15 and 20 times, respectively, as much as that in untreated rats at the maximal levels of induction. The 9000 x g supernatant-mediated mutagenicities of benzo(a)pyrene and 3-methylcholanthrene varied in parallel with the content of MC-P-448 in microsomes, and the induced mutagenicity of benzo(a)pyrene was completely inhibited by anti-MC-P-448 immunoglobulin, o-Aminoazotoluene induced both MC-P-448 and PB-P-450 about 10 and 5 times as much, respectively, while the induced mutagenicity of o-aminoazotoluene was inhibited 90 and 10% by the antibodies to MC-P-448 and PB-P-450, respectively. Both MCP-448 and mutagenic activity of 2-acetylaminofluorene were induced about three times by a single dose of 2-acetylaminofluorene to rats. Administration of aflatoxin B1 showed neither induction of cytochrome P-450 nor an increase of mutagenic activity of aflatoxin B~. It is concluded that there is a good correlation between the increase of mutagenic activity and the 1 This work was supported in part by a Grant-in-Aid for Cancer Research from the Japanese Ministry of Education, Science, and Culture. 2 To whom requests for reprints should be addressed. Received May 25, 1982; accepted November 3, 1982. contents of the two cytochrome P-450 species in microsomes by administration of various carcinogens to rats.