We now report the isolation and the preliminary characterization of a macro-molecular component which has the attributes of the specific receptorfor estrogens. Methods and Materials.-Immature Holtzman rats (20-23 days old) were injected intraperitoneally with various amounts of chromatographically pure 6, 7-H3-estradiol-17f3 (140 or 150, c/, ug, New England Nuclear) in 0.5 ml of 0.9 per cent NaCl-2 per cent ethanol. In some experiments, chromatographically pure estra-diol-17a, corticosterone, testosterone, or diethylstilbestrol was injected in thesame manner. The rats were decapitated 20 min to 4 hr after the injection. The uteri were removed immediately and placed on ice. Groups of five uteri were homogenized together in 0.5 mlof buffer [0.01 M Tris (tris [hydroxymethyllaminomethane) and 0.0015 M EDTA (ethylenediaminetetraacetic acid), pH 7.4, or 0.04 M Tris-0.004 M MgCl2-0.1 M KCI, pH 7.4]. The homogenates were centrifuged at 105,000 X g for 1.5 hr to remove particulate material. Tris-EDTA was found to be more suitable for removing particulate material. The soluble supernatant fractions (0.2 ml) were layered on 4.4-ml gradients of 5-20 per cent sucrose in 0.01 M Tris buffer, pH 7.4, and were centrifuged for 9-12 hr at 39,000 rpm in a Spinco model L using a SW-39 rotor (method of Martin and Ames, 1961). These operations were done at 0-40C. The tubes were pierced and the eluate was run through a Beckman DB flow cell to obtain optical density readings at 280 mis. The eluate was collected in a fraction collector (0.2+ 0.01 ml/fraction), 3 ml of ethanol and 10 ml scintillation

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