We have proposed a multichain model for the high-affinity interleukin-2 (IL-2) receptor involving two IL-2-binding peptides, a 70/75 kilodalton (kD) and a 55 kD, reactive with the anti-Tac monoclonal antibody, which are associated in a receptor complex. With the use of coprecipitation analysis, radiolabeled interleukin-2 cross-linking procedures, and flow cytometric resonance energy transfer measurements, a series of additional peptides of molecular weight 22,000, 35,000, 40,000, 75,000 (non-IL-2 binding), 95,000-105,000, and 180,000 has been associated with the two interleukin-2-binding peptides. In contrast to resting T cells, the abnormal T cells of patients with human T-cell lymphotropic virus I-associated adult T-cell leukemia, patients with select autoimmune disorders, and individuals rejecting allografts express the Tac peptide (p55) of the IL-2 receptor. To exploit this difference in Tac antigen expression, we have initiated therapeutic trials using unmodified anti-Tac, conjugates of anti-Tac with truncated Pseudomonas exotoxin PE-40, interleukin-2-truncated toxin fusion proteins, and alpha- and beta-emitting isotopic chelates of anti-Tac. Furthermore, by genetic engineering humanized hyperchimeric anti-Tac molecules have been prepared in which the molecule is entirely human IgG1, except for the small complementarity-determining regions that are retained from the mouse antibody. This "humanized" antibody manifested the ability to perform antibody-dependent cellular cytotoxicity absent in the original mouse monoclonal. The clinical application of anti-interleukin-2 receptor-directed therapy represents a new perspective for the treatment of certain neoplastic diseases and autoimmune disorders and for the prevention of allograft rejection.