Various studies have shown the mutagenicity of red wine. The major mutagens identified in red wine have been flavonoids, i.e. rutin and its aglycone quercetin. Besides flavonoids, however, it has recently been reported that H 2 O 2 may account for the mutagenicity of red wine in the L-Arabinose resistance test. In the present study we report on the role of flavonoids in the mutagenicity of red wine in the Ames assay. Different wines from Portugal and Spain have been tested after concentration in XAD-2 columns in strains TA98 and TA104 of Salmonella typhimurium concurrently with the determination of the respective content of quercetin by HPLC. A similar approach was used for pilot scale productions of red wines. In all cases quercetin could be demonstrated as the major mutagen in red wines. The levels of quercetin in finished wines and during the wine-making process showed a good fit with the levels of mutagenicity detected. Catalase had no effect whatsoever on the mutagenicity of wines in both TA98 and TA104. These results do not rule out a role for H 2 O 2 in the mutagenicity of wines, detected in other genetic end-points, because H 2 O 2 can be formed from the autooxidation of quercetin.