Cytochrome P-450 was induced both in the diploid wild-type D7 strain and in two isogenic DNA-repair-deficient strains (rad3 and rad56) of Saccharomyces cerevisiae following UV- and X-irradiation. The induction occurred only in logarithmic growth phase cells and it was transient showing a peak 3 h after irradiation. The maximal amount of cytochrome P-450 was directly proportional to the radiation dose applied. Under the same experimental conditions an increase of the catalase activity was also observed, suggesting that activated oxygen species produced by irradiation might be implicated in the induction of both enzymes. The sensitivity to H2O2 of cells containing high cytochrome P-450 levels was enhanced when this enzyme was specifically inhibited by tetrahydrofuran and metyrapone. This supports the hypothesis that cytochrome P-450, as well as catalase, might be involved in cell protection against oxidative damage.