The estrogen receptor has been purified to homogeneity from calf uterus cytosol by sequential affinity chromatography by using heparin--Sepharose 4B and 17-hemisuccinyl-17beta-estradiol-ovalbumin--Sepharose 4B. The procedure yields about 1.2 mg of receptor protein from 1 kg of calf uteri, with a recovery of 53%. The receptor protein, as a complex with 17beta-[3H]estradiol, is purified more than 99%. A single band is seen on polyacrylamide gel ectrophoresis under nondenaturing conditions. 17beta-[3H]Estradiol comigrates with the protein band. As computed from the specific activity of radioactive hormone, 64,450 g of purified receptor protein binds 1 mol of 17beta-estradiol. 17beta-[3H]Estradiol bound to the protein is displaced by estrogenic steriods but not by progesterone, testosterone, or cortisone. As judged by chromatography on calibrated Sephadex G-200 columns, the purified receptor is identical with native receptor in crude cytosol: both show a Stokes radius of 6.4 nm. On sucrose gradient in low-salt buffer, the purified receptor sediments at 8 S. On electrophoresis in NaDodSO4 gels, the purified receptor migrates as a single protein band with an apparent molecular weight of 70,000. The sedimentation coefficient measured on sucrose gradients in the presence of chaotropic salts [1 M NaBr or NaSCN (0.1 M)] is 4.2 S. We conclude that the estrogen receptor of cytosol consists of a single subunit weighing about 70,000 daltons and endowed with one estrogen binding site. Under native conditions in cytosol, several subunits associate to form a quaternary structure with a Stokes radius of 6.4 nm.