PIP: A total of 14 premenopausal human uteri were transferred within minutes from the operating room to the laboratory on ice. All endometrial sampels were in the early proliferative phase. Portions of leiomyomata and of normal myometrium were examined histologically. Other neoplastic and normal portions were homogenized separately. To obtain the cytosol the filtrate was centrifuged twice at high speed. The cytosol was immediately used for sucrose gradient centrifugation, chromatography electrophoresis, and Scatchard analysis. Measurement of the estradiol binding site concentration was done with the charcoal technique. Protein content of the samples was determined, using bovine serum albumin as the standard. Both leiomyomata and normal human myometrium, in each case from the same tumor-bearing uterus, contained specific estrogen receptors in the cytosol. Their affinities for labeled estradiol and electrophoretic mobility after agar gel electrophoresis were similar. Sucrose gradient sedimentation coefficients and chromatographic patterns after agarose gel filtration were identical. In high salt buffer the (tritiated)-estradiol cytoplasmic binding components sedimented as a 4S peak on sucrose gradient. In low salt medium the (tritiated)-estradiol sedimented at about 4S and 8S. Results are in agreement with those of other authors and are consistent with the clinical observation that leiomyomata are estrogen-sensitive tumors.