Quercetin has been extensively studied in various short-term assays for genotoxicity. The patterns of genotoxicity of quercetin for different genetic endpoints are subject to a variety of factors (pH, antioxidants, metabolism) whose precise role in each test remains unclear. In the present study we report on the possible effect of oxygen-derived species on the activity of quercetin in the Ames assay and in the SOS chromotest. Our results seem to suggest that superoxide dismutase (SOD) does not account for the levels of mutagenicity detected in the presence of S9 or S100. The latter may, however, contain other factors of antioxidant defense which may prevent the oxidative degradation of quercetin. Since this degradation occurs at pH values above neutrality and the SOS-inducing activity is higher at pH 6.0, it is concluded that the response of quercetin in the SOS chromotest is due to quercetin itself at acidic pH. The SOS-inducing activity at pH 7.4 is enhanced by SOD, but it cannot be unambiguously concluded that this effect in the SOS chromotest might only be due to protection against the oxidative degradation of quercetin.