The effects of antibodies to cytochromes P-45017 alpha,lyase and P-450C21 on benzo[a]pyrene hydroxylase activity were measured in microsomes from guinea pig adrenals. Anti-cytochrome P-45017 alpha,lyase IgG inhibited about 30% of the benzo[a]pyrene hydroxylase activity of the microsomes in the presence of excess amounts of the IgG, but anti-cytochrome P-450C21 IgG did not affect the activity. In a reconstituted system, consisting of cytochrome P-450, NADPH-cytochrome-P-450 reductase and dilauroylphosphatidylcholine, cytochrome P-45017 alpha,lyase catalyzed, in addition to steroid hydroxylation, benzo[a]pyrene hydroxylation, but cytochrome P-450C21 did not hydroxylate benzo[a]pyrene. Since anti-cytochrome P-45017 alpha,lyase IgG inhibited benzo[a]pyrene hydroxylase activity completely in the reconstituted system with cytochrome P-45017 alpha,lyase, the presence of non-inhibited benzo[a]pyrene hydroxylase in the microsomes suggests that the residual activity in the microsomes may be due to some enzyme other than cytochrome P-45017 alpha,lyase. Benzo[a]pyrene hydroxylase activity was detected in detergent-solubilized microsomes from which cytochrome P-45017 alpha,lyase was removed by using an immobilized anti-cytochrome P-45017 alpha,lyase IgG-Sepharose column. This shows the existence in microsomes of a benzo[a]pyrene hydroxylase other than cytochrome P-45017 alpha,lyase. Benzo[a]pyrene hydroxylase in the solubilized microsomes required O2, NADPH and NADPH-cytochrome-P-450 reductase for its activity and was inhibited by CO. This suggests that benzo[a]pyrene hydroxylase is a cytochrome P-450-dependent monooxygenase. This novel enzyme was also active in xenobiotic metabolism, such as 2-nitropropane denitrification and aminopyrine demethylation.