Objective: In this study, we observed the effect of melatonin on the frequency of sister chromatid exchange, micronucleus formation of binuclear cell in lymphocyte from human peripheral blood in vitro, micronucleus formation of mouse bone marrow polycychromatic erythrocyte in vivo, which were induced by chemical mutagen, and lymphocyte proliferation kenetics in vitro. Methods: ① Lymphocytes were cultured in vitro in the presence of 0.01,0.10,1.00 mmol/L melatonin, mitomycin C(MMC) (positive control), 0.5% ethanol (negative control)and 0.01,0.10,1.00 mmol/L melatonin plus MMC for 72 h at 37℃±1℃. Lymphocytes were examined for the frequence of SCE, mitotic index, cell proliferation cycle, cell cycle ratio and proliferation index. ② Lymphocytes were cultured in vitro in the presence of 0.01,0.10,1.00 mmol/L melatonin, mitomycin C(MMC) (positive control), 0.5% ethanol (negative control) and 0.01,0.10,1.00 mmol/L melatonin plus MMC for 44 h at 37℃±1℃. Then each culture was given cytochalasin B, which was cultured to 72 h. Binuclear lymphocytes were examined for the micronucleus rate. ③ The mice were administered with 0.1, 1.0,10.0 mg/kg*bw melatonin and distillated water (negative control) respectively for 7 d, then were given melatonin plus cyclophosphamide (CP) (positive control) for 2 d since the eighth day. The rate of micronulclei of mouse bone marrow polycychromatic erythrocyte was examined. Results: ① The frequences of sister chromatid exchange of lymphocytes which were cultured in the presence of 0.01,0.10,1.00 mmol/L melatonin compared with negative control exhibited no statistical significance. ② The SCE of cells treated with melatonin plus MMC compared with positive control were markedly decreased. ③ The mitotic indices of lymphocytes cultured in the presence of 0.10,1.00 mmol/L melatonin were lower than negative control. The proliferation index was significant lower than negative control only in the culture exposed to 1.00 mmol/L melatonin. ④ The proliferation cycle of lymphocyte cultured in the presence of 1.00 mmol/L melatonin exhibited an increase in the percent of cells in their first division, and concomitant significant decrease in their third or later division. which were increased in their second division in cells treated with melatonin of three concentration respectively. The ratio between the first and third or later division, between the second and third or later division of lymphocytes exposed to 1.00 mmol/L melatonin were higher than negative control. ⑤ The micronucleus rates of binuclear lymphocytes treated with 0.01,0.10,1.00 mmol/L melatonin plus MMC were lower than positive control, of which the inhibitory rates of micronuclei were 14.37%, 22.17% and 31.34%, respectively. ⑥ The micronucleus rates of mouse bone marrow polycychromatic erythrocytes exposed to 0.1,1.0,10.0 mg/kg*bw melatonin plus CP were lower markedly than positive control, in which it exhibited a significant and concentration－depended decrease, of which the inhibitory rates of micronuclei were 28.89%, 47.73% and 69.96%, respectively. Conclusion: In vitro assay. It seems that melatonin has not only no genotoxic, but also anti-mutagenic effect on lymphocyte. It can inhibit the increase of frequency of SCE in lymphocyte and micronucleus rate of binuclear lymphocyte induced by MMC, and micronucleus rate of mouse bone marrow induced by CP at some concentiations. It can inhibit the lymphocyte proliferation of mitogen stimulated and has some influence on cell proliferation kenetics at higher concentrations (1.00 mmol/L).