Treatment of uterine cytosol with calcium ions under conditions of high ionic strength to deaggregate the estrogen receptor into sub-units yields a ‘stabilized’ 4 S binding unit which does not revert to the 8 S form at low ionic strength and which is resistant to aggregation during purification by ammonium sulfate precipitation, gel filtration and ion-exchange chromatography. By this sequence of steps, the calcium-stabilized estradiol-receptor complex from high speed calf uterine cytosol has been purified about 5000 fold and that from low speed cytosol 1000–1500 fold. Further purification of the latter material by disc gel electrophoresis on polyacrylamide gave a product of high purity, showing a single protein band on analytical disc gel electrophoresis and a sharp 4.5 S sedimentation peak of bound estradiol on sucrose gradient centrifugation.