We have analysed the distribution and the binding properties of the epidermal growth factor (EGF) receptor in human endometrium during the menstrual cycle to validate the proposal that steroids affect the growth rate of target cells via growth factor-related pathways. The total number of EGF receptors and their affinity for the iodinate exogenous ligand are significantly higher during the proliferative than the secretory phase; the binding kinetics appear to be hyperbolic and sigmoid during the proliferative and the secretory phase, respectively. These results suggest that endogenous sex steroids can regulate the levels and the properties of EGF receptors in vivo, and support the possibility that receptor-coupled events may play a role in estrogen-stimulated growth.