Two intracellular high-affinity, low-capacity estrogen-binding proteins, which have the characteristics of receptors, with equilibrium dissociation constants of 10(-10)M and 10(-9)M, have been observed in the human uterus. The higher-affinity protein (10(-10)M) appears to play the main role in activating end-organ response to estrogen stimulation. The role of the lower-affinity protein (10(-9)M) is uncertain. In a human postmenopausal uterine system without estrogen stimulation, Scatchard analysis of uterine cytosol partially purified by ammonium sulfate fractionation and incubated at 4 degrees C for 18 hours revealed only the higher-affinity receptor component (10(-10)M). In a post menopausal uterine system with estrogen priming (and in the premenopausal uterus) both the high- and low-affinity components were observed. Competition studies indicated that the receptors were specific for estradiol. The clinical significance of these findings is discussed.