Two antimycotic agents, the azole ketoconazole and the allylamine terbinafine (Lamisil), have been examined for their effects on the metabolism of tolbutamide, ethinyloestradiol and cyclosporin by human liver microsomes (n = 4) in vitro. Ketoconazole caused marked inhibition of all enzyme activities with mean IC50values (concentration producing 50% inhibition) of 17.9μM (tolbutamide hydroxylase), 1.9μM (ethinyloestradiol 2–hydroxylase), 2.0μM (cyclosporineN-demethylase) and 2.1μM (cyclosporine hydroxylase). At 50μM terbinafine concentration, inhibition was less than 5% for tolbutamide, approximately 12% for both cyclosporin pathways and 30% for ethinyloestradiol. Terbinafine does not have the same inhibitory potential for cytochrome P-450 isozymes as ketoconazole.