In order to determine the binding dissociation constant (KI) of a nonradiolabeled compound, the method of Cheng and Prusoff (1973) (Biochem. Pharmacol. 22, 3099-3108) has been widely employed. The technique involves monitoring the competitive displacement from receptors of a radiolabeled compound with a known dissociation constant, KD, by a nonradiolabeled compound. The concentration of nonradiolabeled compound which displaces 50% of the radiolabel, the IC50, is used to estimate its KI with correction for competition with the radiolabel. The error of this method is small as long as the concentration of receptor is much less than KD and KI, constraints which are sometimes difficult to meet. In this report formulae are derived from which KI can be calculated from IC50 with no constraints on the receptor concentration.