The role of estrogen and estrogen-related growth factors in the mechanism of hormone dependency of endometrial adenocarcinoma cells was investigated. The proliferation of hormone-responsive human endometrial adenocarcinoma cells (Ishikawa cells), which possess both estrogen and progesterone receptors, was optimally stimulated by 10 nM estradiol. Both transforming growth factor (TGF)-α and epidermal growth factor (EGF), added to the culture media, stimulated the proliferation of Ishikawa cells in a dose-dependent manner. Anti-TGF-α antibody completely eliminated the stimulatory effects of TGF-α. Anti-EGF receptor antibody inhibited the proliferation of these cells. The production of TGF-α into culture media was 5-40 pg/10 cells/24 h in 9 human endometrial adenocarcinoma cells. Ten nanomoles of estradiol increased the TGF-α production of Ishikawa cells by approximately 2.5-fold of the control level. In contrast, the production of TGF-α in hormone-unresponsive HEC-50 cels was not influenced by estradiol. C-erbB-2 oncoprotein expression of human endometrial adenocarcinoma cells, detected by both immunocytochemical staining and Western blot analysis, was associated with the tumor grade of the original tumor tissues. Ten nanomoles of estradiol clearly increased the c-erbB-2 oncoprotein levels at an optimal incubation period of 72 h, whereas estradiol did not affect the expression in HEC-50 cells.