The objective of the present study was to evaluate possible genetic changes in cultured human lymphocytes treated with estradiol, using the cytokinesis block micronucleus assay. Eight experimental concentrations of estradiol were used (range from 10(-10) M to 0.7 x 10(-4) M). The obtained results indicate that estradiol exhibits aneugenic and/or clastogenic effects, expressed as increased frequency of micronucleated lymphocytes at two highest experimental concentrations used in this investigation. In addition to genotoxic effects, these concentrations decreased the cytokinesis block proliferation index (CBPI) and percentage of binucleated cells, indicating the cell cycle delay and possible cytotoxic effects. In conclusion, estradiol treatment might represent a human health risk, especially if overdosed or used for a prolonged period of time.