In this study, possible genotoxic effects of zinc oxide (ZnO) nanoparticles were investigated in cultured human peripheral lymphocytes by using chromosome aberrations and micronucleus assays (MN). For this purpose, the cells were treated with ZnO (1, 2, 5, 10, 15 and 20 mu g/mL) for 24 and 48 h. In this research, four types of chromosome aberrations were observed as chromatid and chromosome breaks, fragment and dicentric chromosomes. ZnO induced significant increase of the ratio of chromosomal aberrations as well as percentage of abnormal cells at concentrations of 1, 5, 10 and 20 mu g/mL in 24 h treatments. In 48 h treatments, while ZnO nanomaterials induced significant increase of the percentage of abnormal cells only at a concentration of 10 mu g/mL, and of chromosome aberration per cell in comparison to the control at concentrations of 5 and 10 mu g/mL. On the other hand, this material significantly increased the micronuclei frequency (MN) at concentrations of 10 and 15 mu g/mL in comparison to the control. Cytokinesis-block proliferation index was not affected by ZnO treatments. It also decreased the mitotic index in all concentrations at 24 h but not at 48 h. The present results indicate that ZnO nanoparticles are clastogenic, mutagenic and cytotoxic to human lymphocytes in vitro at specific concentrations and time periods.