AIM:To understand the effect of arsenic trioxide (As2O3) on cultured A375 melanoma cell growth and glutathione peroxidase activity.METHODS:Using methyl thiazolyl tetrazolium (MTT) reduction to assay the inhibition effect of 1.5,3.0,6.0,12.0,24.0μmol/L As2O3 on A375 melanoma cells for 24,48,72 hours,respectively. To determine the glutathione peroxidase activity of the A375 melanoma cells co-cultivated with As2O3 for 24 hours. RESULTS:The growth of A375 tumor cells were inhibited by dose-depending As2O3 significantly. The higher As2O3 concentration was,the lower A375 cells proliferation was. 24 hours later,the glutathione peroxidase activity of the A375 cells was 2.23±0.81,2.87±0.70,4.08±0.81,2.63±0.84,1.40±0.41mU/mL respectively,and the 6.0μmol/L As2O3 had the highest activity. Below or above this concentration,the enzyme activity showed decreasing trend. After statistically independent samples t-tests analysis,the difference was statistically significant.CONCLUSION:As2O3 can inhibit the growth of A375 tumor cells in a dose-dependent manner and can reduce the A375 tumor cell glutathione peroxidase activity significantly.