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The metabolism of benzo[a]pyrene (B]a[P) and its proximate mutagen B[a]P-7,8-dihydrodioI (7,8-diol) was investigated in the presence of human microsomal epoxide hydrolase and P450, 1A1, 1A2, 2C8, 2C9, 2C18, 2C 19, 2D6 and 3A4 expressed in the yeast Saccharomyces cerevisiae. P450, 1A1 had the highest turnover numbers for the formation of all B[a]P metabolites, including phenols and dihydrodiols. P450, 1A2, 2C8, 2C9, 2C18, 2C19 and 3A4, which are well represented in the liver, gave rise to the formation of appreciable amounts of 3-hydroxy-B[a]P and of some dihydrodiols from B[a|P. When 7,8-diol was used as substrate, P450 1A1 also exhibited the highest turnover numbers for the formation of tetrols, the hydrolysis products of the diolepoxides, whereas P450, 1 A2, 2C8, 2C19 and 3A4 showed moderate activities. In order to test the validity of the yeast system, the contribution of each P450 isoform to B[a]P and 7.8-dioI metabolism was evaluated as the product of the turnover numbers of recombinant P450s by specific contents of each P450 in human liver microsomes. Calculated formal ion rates for each B[a]P and 7.8-diol metabolite globally matched experimental values. There is evidence that P450 3A4 and 2C9 play a major role in the formation of 3-hydroxy-13[a]P from B[a]P. Accumulation of the proximate mutagen 7,8-diol was predicted to be mainly driven by P450, 1A2, 2C9 and 2C19, while formation of the genotoxic diolepoxides from 7,8-diol appeared to be dependent on P450, 1A2 and 3A4 in the liver.