A procedure has been developed for purification of a special rat liver estrogen-binding protein. It includes protein precipitation by ammonium sulfate, gel filtration, ion exchange chromatography, and affinity chromatography on estradiol sepharose. The protein is purified 2260-fold with a 27% yield. Upon electrophoresis in 10% PAG in the presence of sodium dodecylsulfate the protein gives one polypeptide strip with a molecular weight of 31.200 +/- 400 dalton.