Cimetidine binds to cytochrome P-450 and inhibits hepatic metabolism of various drugs in humans. However, cytochrome P-450 is a family of enzymes rather than a single protein, and effects of cimetidine on individual human liver cytochromes P-450 have not been previously characterized. Metabolism of selected substrates and cimetidine-binding assays have been performed using human liver microsomes, purified human liver cytochromes P-450, and cytochrome P-450 complementary DNA-expressed yeast proteins to probe interaction of cimetidine with these individual enzymes. Cimetidine (3.0 mmol/L) in incubations reduced bufuralol hydroxylase activity by 80% and strongly inhibited microsomal nifedipine oxidation (23% +/- 13% of control activity). The same concentration of cimetidine produced intermediate inhibition of cytochrome enzymes responsible for ethoxyresorufin deethylation and aniline hydroxylation (77% +/- 6% and 68% +/- 17% of activity in control microsomal incubations, respectively), but little effect on tolbutamide hydroxylation was observed. Concordantly, the calculated binding constant for the binding of cimetidine to a purified cytochrome P-450 with high tolbutamide hydroxylase activity was 4.4 mmol/L, whereas the calculated binding concentration constant for a purified cytochrome P-450-metabolizing nifedipine was 0.7 mmol/L. These studies show a high variability in the effect of cimetidine on drug metabolism by individual human liver cytochromes P-450. In vitro studies using human liver microsomes and genetically engineered human cytochromes P-450 can be very useful in exploring important clinical questions of hepatic drug metabolism.