An established cell culture system of isolated human endometrial stromal and epithelial cells has been used to study the effects of oestrogen and progesterone, as well as their antagonists, upon endometrial cells. Normal hormonal regulation in vivo was investigated simultaneously in endometrial tissue samples taken at different phases of the menstrual cycle. Several marker molecules analysed by immunohistochemistry appeared to depend strongly on endocrine regulation and could be traced in culture. Immunohistochemically, basic parameters of cell biology were identified in vitro, e.g. cell proliferation (Ki-67), adhesion molecules (beta3 integrin) and paracrine factors (leukaemia inhibitory factor). The most reliable parameters to assess hormonal influences were oestrogen and progesterone receptor molecules. Immunohistochemical localization could be improved by molecular biological analysis using RT-PCR. In the presence of oestrogen, a significant expression of hormone receptors was also shown by RT-PCR, and withdrawal of oestrogens and addition of gestagen, i.e. medroxyprogesterone acetate, caused receptor downregulation. Addition of the anti-oestrogen ICI 182.780 to cell-culture medium significantly decreased the synthesis of progesterone receptors.