Interleukin-2 (IL-2) is a lymphokine synthesized by T cells following activation. Resting T cells do not express IL-2 receptors, but receptors are rapidly expressed on T cells following interaction of the antigen-specific T-cell receptor complex with appropriately processed and presented antigens. Anti-Tac, a monoclonal antibody that recognized the IL-2 receptor, has been used to purify the receptor. The recognized the IL-2 receptor, has been used to purify the receptor. The receptor is a 55-Kd glycoprotein comprised of 272 amino acids including a single 19-amino transmembrane domain and a short intracytoplasmic domain composed of 13 amino acids at the carboxy terminus. Normal resting T cells and most leukemic T-cell populations examined did not express IL-2 receptors; however, the leukemic cells of all patients with human T-cell lymphotrophic virus (HTLV-I)-associated adult T-cell leukemia (ATL) expressed the Tac antigen. In HTLV-I-infected cells, the 42-Kd long open reading frame (tat) protein encoded in part by the tat region of HTLV-I may act as a transacting activator that induces transcription of the IL-2 receptor gene, thus providing an explanation for the constant association of HTLV-I infection of lymphoid cells and IL-2 receptor expression. The constant display of large numbers of IL-2 receptors which may be aberrant in the ATL cells may play a role in the uncontrolled growth of these leukemic T cells. Patients with the Tac-positive ATL are being treated with both unmodified and toxin-conjugated forms of anti-Tac monoclonal antibody directed toward this growth factor receptor.