Radiolabeled molecules from detergent-solubilized human T cell blasts were fractionated on affinity supports coupled with T cell growth factor (TCGF) and anti-Tac antibody. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis indicated that a glycoprotein of approximately 58,000 mol wt was bound in both cases. Sequential binding to the two affinity supports demonstrated that the molecules recognized in each instance were identical. Thus, the Tac antigen contains the cellular binding site for TCGF. Note added in proof: Preliminary binding experiments using high concentrations of [3H]leucine, lysine TCGF with a low specific radioactivity indicate the existence of a sizeable pool of receptor sites with an affinity 2,000-10,000 times lower than that of the high affinity receptors measured in Fig. 1. Such sites may explain the numerical discrepancy between early TCGF binding experiments (5) and the binding of the anti-Tac antibody. The hypothesis that the anti-Tac antibody apparently reacts with both classes of receptor would explain its effect on the physiological response (high affinity binding) and the high level of Tac antigen on the cell surface.