Isolation of the estrogen receptor was undertaken for comparison with the progesterone receptor with respect to seasonal variations in receptor level and function and the specificity of nuclear binding sites in the hen oviduct. The estrogen receptor in the chick and hen oviduct has been only partially characterized, with conflicting or incomplete information on its properties. We have isolated a binding protein and have partially purified it by ammonium sulfate precipitation. The binding protein displays a single, high affinity class of sites (Kd similar to 10(-10) M) with a low capacity (35 fmol/mg protein) for 17beta-estradiol, shows a marked specificity for estrogenic compound, and is tissue specific. The protein sediments at approximately 8S in low salt and at approximately 4S in high salt conditions. The protein elutes from molecular sieve chromatography (agarose-0.5 m) in high salt conditions (0.3 M KCI) in a molecular weight range of approximately 60,000. DEAE chromatography and isoelectric focusing suggest that there are two molecular species, one focusing at pH 6.8 and the other at pH 7.3. Estrogen complexed to this protein will bind to nuclear acceptor sites in a cell-free assay in which free estrogen, estrogen bound to serum proteins, and [3H]estradiol receptor exchanged with a 100-fold excess of cold estradiol do not bind. Binding to nuclei is saturable and displaceable by an excess of nonradioactive estrogen receptor complexes under conditions of constant protein concentration. The above results represent the most detailed characterization to date of the cytosol estrogen receptor of hen oviduct This is the first demonstration of saturable, receptor-dependent, cell-free binding to target organ nuclei.