The cortisol binding capacity was decreased by trypsin digestion of the lens protein. The binding capacity was decreased by the addition of 8.0 m of urea and restored partially by dialysis, suggesting that the binding of cortisol to the lens protein is reversible and that thiohydryl groups and hydrogen bonds are important as a binding site. The binding of cortisol by lens protein was quite rapid, reaching a maximum level within 60 sec. The binding of cortisol by lens protein almost vanished when the lens was frozen. Binding of cortisol-4-[<sup>14</sup>C] to the lens protein was inhibited competitively by unlabelled cortisol, and the binding of cortisol-4-[<sup>14</sup>C] to the lens protein showed saturation by added cortisol-4-[<sup>14</sup>C]. The cortisol-binding capacity was markedly less in the matured senile cataractouslens because there was a loss of β-crystallin.