This study examined the effects of estrogen, progesterone and tamoxifen at different concentrations and treatment periods on proliferation of a human cervical carcinoma cell line, HeLa, in culture, based on MTT assay. Estrogen did not have an effect on the cellular proliferation in concentrations up to $1{mu}g$/ml for treatment periods of between 2.5 and 6 days, but significantly inhibited proliferation at a higher concentration of $10{mu}g$/ml in a progressive manner with increasing treatment periods. Also, treatment of HeLa with more than $10{mu}g$/ml of progesterone for 2.5 days significantly inhibited proliferation and caused a concentration-dependent inhibition with 4 days of treatment. However, longer treatment with progesterone for 6 days abolished the concentration-dependent inhibitory effect on cellular proliferation observed with the 4-day treatment period. Furthermore, tamoxifen required a higher concentration ($100{mu}g$/ml) than estrogen to bring about the inhibitory effect on the HeLa proliferation. These results suggest that high concentrations of estrogen, progesterone and tamoxifen may suppress proliferation of HeLa, and both the concentration and the treatment period may also influence their inhibitory effects on cellular proliferation.