A supernatant protein fraction prepared from rat liver cytosol has been shown to specifically bind corticosteroid. This binding has been investigated under various conditions using Sephadex G-50 and G-200 chromatography. Ion exchange chromatography has been used to distinguish the receptor-binding protein from transcortin which is present to a variable extent in our liver preparations. The retention characteristics on DEAE-cellulose columns indicate that the receptor-binding protein is more basic than transcortin and has an association constant for corticosterone-binding of the order 108 1/mole. Partial purification of the binding protein has been carried out and an amino acid analysis used to determine a more accurate molecular weight for the fraction.