Over 70 naturally occurring and synthetic flavonoids were screened for mutagenicity with 5 tester strains in the Salmonella/mammalian microsome assay: TA1535, TA100, TA1537, TA1538 and TA98. Frameshift mutagenicity was confined to the flavonols (flavon-3-ols) in strain TA98, TA1537 and TA100. The