Ig and T cell antigen receptor genes in their germ line form are separated DNA segments that are joined by recombinations during lymphocyte development. The analysis of Ig and T cell receptor gene arrangements has been of value in the study of lymphoid neoplasms. The identification of T cell receptor gene rearrangements taken in conjunction with studies of Ig gene rearrangements aids in the elucidation of the lineage (T cell or B cell) and the clonality of lymphoid populations of all series. The application of this molecular genetic approach has great potential for complementing conventional marker analysis, cytogenetics, and histopathology, thus broadening the scientific basis for the classification, diagnosis, and monitoring of the therapy of lymphoid neoplasia. IL 2 is a lymphokine synthesized by some T cells following activation. Resting T cells do not express IL 2 receptors, but receptors are rapidly expressed on T cells following the interaction of antigens, mitogens, or monoclonal antibodies with the antigen-specific T cell receptor complex. Normal resting T cells and most leukemic T cell populations do not express IL 2 receptors; however, the leukemic cells of all patients with HTLV-I-associated adult T cell leukemia examined expressed the Tac antigen. The constant display of large numbers of IL 2 receptors that may be aberrant may play a role in the uncontrolled growth of these leukemic T cells. Patients with the Tac antigen positive adult T cell leukemia are being treated with the anti-Tac monoclonal antibody directed toward this growth factor receptor.