The authors compared the properties of nonreceptor estrogen-binding protein (NREBP) of the ovaries of immature rats, specific estrogen-binding protein (SEBP) of the liver and alpha-fetoprotein (alpha-FP) of the rat blood serum. It was shown that all 3 proteins with moderate affinity bound 3H-estradiol (Ka approximately 10(8)M-1) and were characterized by a high rate of interaction with ligand (K+1 approximately 10(6)M-1c-1) and dissociation of hormone-protein complexes (K-1 approximately 10(-2)c-1). NREBP and alpha-FP manifested a very close hormonal specificity of affinity differing from SEBP specificity: both proteins bound intensively estrone and estradiol, to a lesser degree, estriol, and did not bind androgens and hexestrol. Stokes' radii of the molecules of 3 proteins were 4.4 and 2.5 nm, respectively. Rabbit antiserum to alpha-FP suppressed the hormone-binding activity of blood alpha-FP and ovarian NREBP but not liver SEBP. With growth of animals a study of the time course of the levels of ovarian NREBP and blood alpha-FP has shown that the presence of NREBP in the ovarian cytosol does not probably result from blood proteins mechanical contamination of the sample in homogenization. It has been assumed that intracellular alpha-FP or its close analog may be involved in the regulation of ovarian endocrine function, reversibly binding estrogens.