Properties and isolation possibilities of an estradiol (E2) binding protein from male rat liver cytosol as possible estrogen receptor are studied. The protein is found to differ considerably in a number of characteristics from well-known receptor and non-receptor rat proteins, specifically binding estrogen. Equilibrium constant of E2-protein interaction is 1-10(8) M-1, the concentration of protein binding sites is 1.3 million per cell. Processes of E2 association with protein and the complex dissociation proceed with a very high rate. E2 and, at lesser degree, estrone, testosterone and 5-alpha-dihydrotestosterone compete for binding sites of protein with 3H-E2. Progesterone, corticosterone and a synthetic estrogen, hexestrol, denot compete. The protein studied is thermolabile and sensitive to the effect of pronase and sulfhydryl reagents. Characteristics of the protein are as follows: sedimentation coefficient--3.6S; Stockes' radius--23--25 A; friction coefficients ratio--1.05--1.12; molecular weight--36000--39000. The protein is not revealed in the liver of female rats. Ammonium sulphate precipitation, gel filtration and ionic exchange chromatography made possible to obtain 50-fold purified protein preparation.