Sixty-four flavonoids were tested for their antimutagenic potencies with respect to IQ in Salmonella typhimurium TA98 and in part also towards MeIQ, MeIQx, Trp-P-2, and Glu-P-1 and in S. typhimurium TA100. Antimutagenic potencies were quantified by the inhibitory dose for 50% reduction of mutagenic activity (ID50). A carbonyl function at C-4 of the flavane nucleus seems to be essential for antimutagenicity: two flavanols and four anthocyanidines were inactive. Again, five isoflavons, except biochanin A, were inactive. Within the other groups of 21 flavones, 16 flavonols and 16 flavanones the parent compounds flavone, flavonol, and flavanone possessed the highest antimutagenic potencies (ID50: 4.1, 2.5, 5.5 nmoles). Increasing polarity by introduction of hydroxyl functions reduced antimutagenic potency. Reducing polarity of hydroxy flavonoids by methyl etherification, however, increased antimutagenic potency again. 6-Hydroxy- and 2'-hydroxy substituted flavonoids were considerably less potent antimutagens. Of 11 flavonoid glycosides tested all compounds except apigenin- and luteolin-7-glucoside (ID50:74, 115 nmoles) were inactive or only weakly antimutagenic. Rings C and A of the nucleus were not essential for antimutagenicity: chalcone and three derivatives were nearly as active as comparable flavones while antimutagenicity of benzylidenacetone was considerably reduced (ID50: 95 nmoles). Cinnamylaldehyde and cinnamoates, however, were inactive. A planar structure in the vicinity of the carbonyl group may also be important for antimutagenicity. Flavanones were less potent antimutagens than the corresponding flavones, but dihydrochalcones and 14 structurally related saturated aromatic carbonyl compounds were inactive. Fisetin and 6-hydroxyflavone were competitive inhibitors, but luteolin was a mixed type inhibitor. The inhibition mechanisms of flavone, kaempferol, morin, flavanone, and 2'-hydroxyflavanone were concentration dependent, being competitive at low concentrations and mixed or non-competitive (2'-hydroxyflavanone) at concentrations about the ID50 value. No fundamental differences between the two tester strains and no clear influence of mutagen structure on antimutagenic potency could be detected.