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A progesterone binding protein was characterized in the cytosol of human myometrium. The receptor sedimented in the 4S region on sucrose gradients and had an equilibrium dissociation constant of 3.7 times 10-9M. Synthetic progestagens had an affinity equivalent to that of progesterone while cortisol had low affinity for the binding protein. These same progestagens showed very low binding to human CBG. In normally cycling women, the total levels of cytosol receptor were higher during the follicular phase than during the luteal phase. Subjects taking oral contraceptives and those who were pregnant had the lowest concentration of myometrial progesterone receptor. Postmenopausal women had levels of myometrial receptor lower than those of the secretory phase of the cycle. Estrogen therapy increased the receptor concentrations of the menopausal myometrium to those of the proliferative phase. The estradiol receptor levels of myometrial cytosol revealed similar changes in cycling women. When both binding proteins were measured concurrently in the same specimen, there was a significant positive correlation.