Carcinogenic aromatic amines such as amino acid and protein pyrolysate compounds and aminoazo dyes induced predominantly P450IA2, which is responsible for mutagenic activation of the aromatic amines, in rats and mice. The induction rate and total activity of this cytochrome P-450 isozyme were well correlated with sex, species, and target organ differences in hepatocarcinogenic susceptibility of animals to the aromatic amines. Preneoplastic liver cells induced by treatment with N,N-diethylnitrosamine and 2-acetylaminofluorene diminished the capacity for P450IA2 induction but retained that for P450IA1 induction. These findings confirm that P450IA2 has an important role in hepatocarcinogenesis by aromatic amines.