Abstract Yeast cytochrome c peroxidase (CCP) efficiently catalyzes the reduction of H 2O 2 to H 2O by ferrocytochrome c in vitro. The physiological function of CCP, a heme peroxidase that is targeted to the mitochondrial intermembrane space of Saccharomyces cerevisiae, is not known. CCP1-null-mutant cells in the W303-1B genetic background ( ccp1Δ) grew as well as wild-type cells with glucose, ethanol, glycerol or lactate as carbon sources but with a shorter initial doubling time. Monitoring growth over 10 days demonstrated that CCP1 does not enhance mitochondrial function in unstressed cells. No role for CCP1 was apparent in cells exposed to heat stress under aerobic or anaerobic conditions. However, the detoxification function of CCP protected respiring mitochondria when cells were challenged with H 2O 2. Transformation of ccp1Δ with ccp1 W191F , which encodes the CCP W191F mutant enzyme lacking CCP activity, significantly increased the sensitivity to H 2O 2 of exponential-phase fermenting cells. In contrast, stationary-phase (7-day) ccp1Δ- ccp1 W191F exhibited wild-type tolerance to H 2O 2, which exceeded that of ccp1Δ. Challenge with H 2O 2 caused increased CCP, superoxide dismutase and catalase antioxidant enzyme activities (but not glutathione reductase activity) in exponentially growing cells and decreased antioxidant activities in stationary-phase cells. Although unstressed stationary-phase ccp1Δ exhibited the highest catalase and glutathione reductase activities, a greater loss of these antioxidant activities was observed on H 2O 2 exposure in ccp1Δ than in ccp1Δ- ccp1 W191F and wild-type cells. The phenotypic differences reported here between the ccp1Δ and ccp1Δ- ccp1 W191F strains lacking CCP activity provide strong evidence that CCP has separate antioxidant and signaling functions in yeast.