Cu,Zn-superoxide dismutase (SOD1) is a critical enzyme in the cellular antioxidant system. The yeast Saccharomyces cerevisiae SOD1 mutant (SOD1D) exhibits a moderate mutator phenotype under aerobic conditions. The mutation frequency of a SOD1D strain determined by a CAN1 forward-mutation assay was about 12-fold higher than that of the parental strain. Base substitutions G･C→ T･A, G･C→A･T, and A･T→C･G were most commonly observed in CAN1 mutants, indicating that the mutations are caused mainly by oxidative DNA damage. The mutation frequency of SOD1D was reduced in a dose-dependent manner by cultivating it in the presence of ascorbic acid, implying that the SOD1D mutant can be used as a tester strain for small molecule antioxidants. Exogenous glutathione and N-acetylcystein also alleviated the mutator phenotype. The results indicate that ascorbic acid and thiol antioxidants are able to e‹ciently protect cells against oxidative damage-induced mutagenesis. In this assay, no apparent mutation suppression was seen for other categories of antioxidants including resveratrol, Trolox and melatonin.