Troxerutin, a flavonoid derivative, used in vascular diseases was studied in 4 mutagenicity tests: the Ames test, the point mutation test (V79/HPRT), the in vitro metaphase analysis in human lymphocytes and the micronucleus test in mice. The aglycone trihydroxyethylquercetin (THEQ) and quercetin were studied too. Troxerutin was not mutagenic, whereas quercetin was positive in the Ames test, V79 cells and in vitro metaphase analysis. THEQ was negative in the Ames test. The substitution of quercetin with hydroxyethyl groups in 7,3' and 4' positions abolished mutagenic activity of quercetin.