Activation of resting T-lymphocytes induces synthesis of interleukin-2 (IL-2) and expression of cell surface receptors for this lymphokine. In contrast to resting normal T-cells that do not express high-affinity IL-2 receptors (IL-2R), abnormal T-cells of patients with leukemia-lymphoma, certain autoimmune disorders, and individuals rejecting allografts express this receptor. Exploiting this difference in receptor expression, antibodies to the IL-2 receptor have been used effectively to treat patients with leukemia and lymphoma. One approach is to use monoclonal antibodies produced in mice; the disadvantage is that they are highly immunogenic. In an effort to reduce the immunogenicity of the mouse monoclonal antibodies, monoclonal-antibody-mediated therapy has been revolutionized by generating humanized antibodies produced by genetic engineering in which the molecule is human except for the antigen-combining regions, which are retained from the mouse. Further, to increase its cytotoxic effectiveness, the monoclonal antibody has been armed with toxins or radionuclides. Alternatively, IL-2 itself has been linked to a toxin to kill IL-2 receptor-bearing cells. Thus, IL-2 receptor-directed therapy provides a new method for treating certain neoplastic diseases and autoimmune disorders and for preventing allograft rejection.