Microsomal hydroxylation of tolbutamide in Japanese livers was studied in vitro to ascertain the enzyme catalysing this reaction. Rates of tolbutamide hydroxylation differed individually 33-fold and 42-fold at 0.1 mM and 2.4 mM tolbutamide concentrations, respectively, and were segregated into two groups, rapid and slow metabolizers. An antibody raised against P450 human-2 (a form of CYP2C9) strongly inhibited the hydroxylation in livers of rapid metabolizers but only weakly inhibited in the slow metabolizer. Kinetic experiments further demonstrated a clear distinction in tolbutamide hydroxylation between two groups; the mean of apparent Km values for tolbutamide was 0.25 mM (n = 3) in the rapid group and 2.58 mM (n = 2) in the slow, respectively. These data suggest that different enzymes are involved in the hydroxylation in both metabolizer groups. Furthermore, CYP2C9 produced by cDNA expression in yeasts, catalysed tolbutamide hydroxylation at rates similar to the rapid metabolizer group at both the 0.1 mM and 2.4 mM concentrations. The apparent Km value of the expressed protein for tolbutamide, 0.26 mM, was similar to that determined for the rapid group of microsomal samples. Clear correlations were observed between the rate of microsomal tolbutamide hydroxylation at 0.1 mM and CYP2C9 protein content or the rate of S-mephenytoin 4'-hydroxylation in human liver. These results indicate that considerable portions of microsomal tolbutamide hydroxylation are catalysed by CYP2C9 or the closely related form in the rapid metabolizers.