Protective effect of myricetin (MRY) against the cytotoxic and genotoxic effects of a hormonal steroid, 17 beta-estradiol (E-2), was assessed in peripheral blood human lymphocyte culture. Sister chromatid exchanges (SCE), mitotic index (MI) and replication index (RI) were scored as genetic endpoints. Firstly, the genotoxic effect of different amounts (5, 10 and 20 mu M final concentration) of E2 was tested, and 10 and 20 mu M E-2 levels were detected as genotoxic. In the second set, E-2 groups were treated with 10 mu M MRY. MRY reduced the SCE but increased MI as well as RI, suggesting its protective action on human lymphocytes in vitro against E-2-induced genotoxic damage.