Antigen or mitogen-induced activation of resting T cells induces the synthesis of interleukin-2 (IL-2) as well as the expression of specific cell surface receptors for this lymphokine. Failure of the production of either IL-2 or its receptor results in a failure of the T-cell immune response. The receptor is composed of a 33,000-dalton (251-amino acid) peptide precursor that is post-translationally glycosylated into the mature 55,000-dalton form. In contrast to resting T cells, human T-cell lymphotrophic virus I (HTLV-I)-associated adult T-cell leukemia cells constitutively express large numbers of IL-2 receptors. Because IL-2 receptors are present on the malignant T cells but not on normal resting cells, clinical trials have been initiated in which patients with adult T-cell leukemia are treated with a monoclonal antibody that binds to the IL-2 receptor.