Genotoxic testing of flavonol glycosides, which account for most of the human intake of flavonoids, is dependent on the use of enzymatic extracts that exhibit β-glycosidic activity. This study was aimed at characterizing further the β-glycosidic activity of cultured cell-free microbial extracts from human faeces (faecalase) and saliva (salivase). Using o-nitrophenyl-β-d-galactoside as substrate, the optimum pH and apparent K m and energy of activation were shown to be 7.6, 3.5 × 10 -4 m and 8.65 kcal/mol, respectively, for faecalase, and 7.4, 8.7 × 10 -5 m and 3.8 kcal/mol, respectively, for salivase. Rutin (quercetin-3-O-rutinoside) was shown to be a competitive inhibitor for faecalase, whereas no inhibitory activity could be found for salivase. Enzymatic hydrolysis of rutin gave the mutagenic product quercetin that was detected in the Ames assay and using high-performance liquid chromatography.