1. Liver nuclei from estrogenized chickens contain high-affinity low-capacity estrogen-binding sites, which are in part salt-extractable (60--70%) and in part tightly sticking to the nuclear residue (30--40%). During the preparation of chromatin with low salt buffers part of the salt-extractable nuclear-binding sites remains together with the non-extractable sites on the chromatin. Extractable and non-extractable sites can be separated by agarose as well as hydroxyapatite chromatography. 2. The extrogen-binding protein extracted from crude nuclei was characterized as follows: sedimentation coefficient of 3.9 S, Strokes' radius (a) of 3.3 nm, molecular weight (Mr) of 56,000 and frictional ratio (f/fo) of 1.20. Trypsination results in a globular receptor fragment (Mr = 41,000), which has lost a small asymmetric portion of the receptor molecule but still binds estradiol. 3. In contrast to the binding protein from crude nuclei the estrogen-binding protein extracted from purified nuclei at pH 7.4 is found mainly in an aggregated form. Dissociation of the aggregates is achieved in high salt/urea resulting in a receptor molecule with an apparent molecular weight of 130,000. Aggregation of the binding sites can be prevented to some extent by raising the pH of the extraction medium. At pH 8.7 the non-aggregated part of the binding protein from pure nuclei could be characterized as follows: 4.4 S, a = 4.3nm, Mr = 80,000 (apparent molecular weight of 150,000), f/fo = 1.40. 4. Mixing experiments indicate that an extranuclear protease present in a crude nuclear preparation converts the large receptor (the binding protein from pure nuclei) to the smaller one (the binding protein from crude nuclei) by digesting off an asymmetric portion of the molecule. This portion seems to be responsible for the strong tendency of the binding protein from pure nuclei to associate with other nuclear components.