Abstract: Objective To observe restrictive impact of Se-methylselenocysteine (MSC) on the growth of human breast cancer cells (MDA-MB-231).Methods MDA-MB-231 cells were challenged with different concentrations of MSC (12.5,25,50,100,and 200 μmol/L)for 24 and 48 hours,respectively.The inhibition rate of MDA-MB-231 cells was assayed with 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) assay.The cell apoptosis was observed with Hoechst stain.The activities of superoxide dismutase (SOD),malondialdehyde (MDA)and glutathione peroxidase (GSH-Px) in MDA-MB-231 cells were determined with test kit.Results MSC can inhibit proliferation of MDA-MB-231 cells.Compared with that of control group(100±0.00%),the survival rate of MDA-MB-231 cells treated with 200 μmol/L MSC decreased remarkably at 24 (64.15±2.81%)and 48 hours (42.57±2.25%).MTT assay showed that MSC could inhibit the proliferation of MDA-MB-231 cells in dose- and time-dependent manner (P<0.01).Hoechst stain demonstrated that there were obvious chromatin condensations in MDA-MB-231 cells of MSC pretreatment group.Compared with those in the control group,the activities of SOD (20.99±3.03 vs.82.47±1.99 U/mg protein) and GSH-Px (22.00±0.75 vs.46.69±0.55 U/mg protein)declined but the activity of MDA (36.20±0.25 vs.5.80±0.11 μmol/mg protein)increased obviously in the MDA-MB-231 cells treated with 200 μmol/L MSC for 24 hours and the variations were also observed in the cells with the same MSC treatment for 48 hours.Conclusion MSC can inhibit proliferation and induce apoptosis in human breast cancer cell line MDA-MB-231 in dose- and time-dependent manner;the mechanism of the inhibitive effect of MSC may related to the change in antioxidant activity.