The mechanism of action of sex steroid hormones on endometrial carcinoma was analyzed by a cell culture system. 1) Estrogenic actions such as enhancement of ALP activity, PR level and cellular growth were confirmed in Ishikawa endometrial carcinoma cells which possess ER. 2) ER and PR are both localized immunocytochemically in the nuclei of Ishikawa cells, confirming the correctness of Gorsky and Greene's theory in endometrial carcinoma. 3) In Ishikawa cells with PR, progestins stimulated E2DH activity and glycogen synthesis and reduced ER level, thus proving the existence of their hormonal action. We cannot, however, rule out the pharmaceutical action of progestin, because a decrease in nucleic acid syntheses and ALP activity, alteration in electron microscopic observation and finally cellular death were observed in endometrial cancer cells without PR. 4) Conversely, Tamoxifen, anti-estrogen, stimulated the cellular growth in serum-free environment of culture. We must therefore be careful in using it clinically.