Liver dextromethorphanO‐demethylation to dextrorphan is associated with the debrisoquin type of oxidation phenotype in humans. We studied dextromethorphan oxidation in vitro using human liver microsomes to investigate the kinetics of the polymorphic monooxygenase (cytochrome P‐450 db1) and factors that may influence its activity. In microsomal preparations from six extensive metabolizers the reaction parameters were: Michaelis‐Menten constant = 3.4 ± SD 1.0 µmol/L and maximum rate of metabolism = 10.2 ± 5.3 nmol × mg P−1× hr−1, vs 48 µmol/L and 2.2 nmol × mg P−1× hr−1, respectively in microsomes prepared from the hver of one poor metabolizer. The reaction was inhibited by nonspecific monooxygenase inhibitors such as SKF 525‐A (Ki = 100 nmol/L) and cimetidine (Ki = 40 µmol/L), by known substrates of the polymorphic isozyme such as [+]‐bufuralol (Ki = 7.5 µmol/L), debrisoquin (Ki = 25 µmol/L), and sparteine (Ki = 45 µmol/L), and by the selective cytochrome P‐450 db1inhibitor quinidine (Ki = 15 nmol/L). This assay permits in vitro screening for candidate substrates or inhibitors of the polymorphic isozyme.Clinical Pharmacology and Therapeutics(1989)4