By means of dextran-coated charcoal assay, the capacity of various endometrial cytosol preparations for specific binding of 3H-rone was determined. With the use of an arbitrary value of 50 femtomoles of bound 3H-progesterone per milligram of cytosol protein as the breaking point between high and low binding capacities, 19 out of 20 normal endometria had high progesterone-binding capacities. Two out of 11 Grade I, 3 out of 8 Grade II, and 2 out or 4 Grade III endometrial carcinomas showed low binding capacities. All 4 endometrial polyps, 7 out of 9 hyperplastic endometria, and 0 out of 7 nonendometrial gynecologic tumors had high binding capacities. These data suggest a progressive loss of specific progesterone-binding activity from normal endometria to hyperplastic endometria, and from the well-differentiated to the anaplastic forms of endometrial adenocarcinoma. There seemed to be an inverse relationship betweeen age and concentration of progesterone receptors in endometrial adenocarcinomas. All irradiated tumors studied had low progesterone-binding capacity.