A form of cytochrome P-450 is produced in Saccharomyces cerevisiae strain D7 during the logarithmic growth phase in 20% glucose liquid medium. This form was inhibited by metyrapone, tetrahydrofuran and by carbon monoxide, specific inhibitors of cytochrome P-450 in mammals. The inhibition was observed by means of the increase of the genetic activity of 4-nitroquinoline 1-oxide (4-NQO) on logarithmic growth phase cells of D7 strain, adding the inhibitors to the incubation mixture. 4-NQO is a strong direct mutagen on stationary phase cells that is detoxified by the monooxygenase system. The inhibition was measured by determining the decrease of the O-deethylation of 7-ethoxycoumarin in whole cells of yeast depending on different concentrations of inhibitors. A decrease of O-deethylation activity was found in the presence of tetrahydrofuran and metyrapone and this behaviour is typical of the cytochrome P-450 species induced by ethanol, as in mammals. Adding sodium phenobarbital to 0.5% glucose liquid medium, a form inhibited only by metyrapone was obtained. The presence of different inducible forms of cytochrome P-450 is evident.