This study examined the effects of tamoxifen at different concentrations and treatment periods on proliferation of a human cervical cancer cell line, HeLa. As in the previous studies, Estrogen did not have an effect on the cellular proliferation at low concentrations but significantly reduced proliferation at high concentration (5 ug/ml) based on MTT assay. Treatment with tamoxifen had similar effects. It did not have any significant effect on the HeLa cell proliferation at low concentrations, but significantly reduced proliferation at high concentration (10 uM). In addition, combined treatment with both estrogen and tamoxifen did not alter the inhibitory effects of either estrogen or tamoxifen on cellular proliferation. The inhibitory effects by tamoxifen on HeLa cell proliferation did not differ among different treatment periods. This suggests that tamoxifen may exert anti- proliferative effects at high concentration but does not have synergistic or antagonistic effects against estrogen on HeLa cell proliferation.