The synthesis of biospecific adsorbents for the purification of the cytosol estrogen receptor from calf uterus is described. The characteristic of several estradiol derivatives, spacer chains, and insoluble matrix were systematically studied. Estradiol derivatives substituted at positions C2, 3, 4 7 alpha, 17 alpha, and 17 beta were tested for their affinities for the receptor; positions 7 alpha and 17 alpha were the most suibable. Acidic compounds had lower affinities than their methylester analogues. Long chain derivatives bound the receptor less firmly than corresponding shorter chains. However, when these ligands were attached to an insoluble matrix, the long spacer chain derivatives (greater than or equal to 14 atoms) were more efficient than the shorter ones. There was a satisfactory parallelism between affinities of free ligands and receptor binding to the respective biospecific adsorbents. On the basis of their stability in the presence of cytosol (no release of ligand), due to the absence of ester bonds, long chains were selected as spacers. Both acrylamide and agarose biospecific adsorbents displayed some ionic exchange capacity and consequently nonspecifically bound proteins; the influence of this nonspecific binding on the purification of the receptor was studied. On the basis of their stability, of their binding specificity, and of their selectivity for the receptor, the estradiol-7 alpha derivative adsorbents were selected for the purification of the receptor.