Activation of resting T cells induces synthesis of interleukin-2 (IL-2) and expression of its specific high-affinity receptor. We proposed a multichain model for the high-affinity IL-2 receptor in which both a 55-kilodalton IL-2-binding peptide identified by the anti-Tac monoclonal antibody and a 70/75-kilodalton IL-2-binding peptide are associated in a receptor complex. The IL-2 receptor is proving to be an extraordinarily versatile therapeutic target, since it is expressed by the abnormal T cells in patients with certain lymphoid malignancies or autoimmune disorders and in individuals rejecting allografts, whereas it is not expressed by normal resting cells. Monoclonal antibodies and toxin-lymphokine conjugates directed toward IL-2 receptors represent novel therapeutic agents for these clinical conditions.