Most techniques for determining cortisol binding characteristics of transcortin and albumin involve disturbance of the equilibrium between cortisol and binding protein. Only ultrafiltration of undiluted plasma with temperature and pH control allows binding constants to be measured under near physiological conditions. Equilibrium dialysis is convenient and widely used for investigating cortisol binding but the interpretation of data has been the subject of controversy (Mills, 1962; Tait & Burstein, 1964). The standard method of calculation (Slaunwhite & Sandberg, 1959) is valid only when related to the final equilibrium position. The derived binding constants refer to cortisol and transcortin concentrations within the sac at the end of dialysis which often differ greatly from those in the original plasma. We describe here a simple method for treatment of dialysis data which, with appropriate correction for dilution, yields binding constants in close accord with those obtained by ultrafiltration of undiluted plasma. Plasmas containing labelled