Glycosides of flavonols such as quercetin, are found in the edible portions of most food vegetables. Flavonols present in plants as glycosides can be freed during fermentation. We have compared the DNA-damaging activity of quercetin, rutin (3-o-rutinoside of quercetin) and a fermented flavonoid-containing beverage, red wine, for different genetic end-points under different metabolic conditions. The genotoxicity of quercetin, rutin and commercial red wine has been studied for the induction of: (i) reverse mutation in the Ames assay; (ii) SOS functions in the SOS Chromotest; (iii) sister-chro-matid exchanges (SCEs) in human lymphocytes. While in the Ames assay the mutagenicity of quercetin is enhanced by the presence of rat liver microsomal enzymes (S9) or the respective cytosolic fraction (S100), genotoxicity is reduced when the induction of SOS responses is assessed using the SOS Chromotest. Similarly, the induction of SCEs is lowered when testing in the presence of liver enzymes. Rutin has no activity whatsoever. Detection of activity of red wine in the three assays is not dependent upon hydrolysis by glycosidases and its content of quercetin accounts almost entirely for the levels of genotoxicity detected. The results suggest that the putative genotoxic metabolites of quercetin vary for different genetic end-points considered and that the metabolic fate of flavonoids might partly account for the conflicting data about their genotoxicity in vivo and carcinogenic activity.