Polyclonal antibodies raised toward a debrisoquine 4-hydroxylating cytochrome P-450 species purified from rat liver (P-450UTA) were used to determine which monooxygenase reactions are linked to debrisoquine hydroxylation in human liver. Anti P-450UTA did not inhibit the oxidation of dimethylnitrosamine, morphine, diazepam, vinylidene chloride, trichloroethylene, benzo(a)pyrene and its 7.8-dihydrodiol, but was inhibitory for the hydroxylation of debrisoquine, (±)-bufuralol, lasiocarpine and monocrotaline. A model interpreting the substrate specificity of the human liver enzyme is presented. .