Recent studies using cloned antigen specific T lymphocytes and monoclonal antibodies directed at their various surface glycoprotein components have led to identification of the human T cell antigen receptor as a surface complex comprised of a clonotypic 90 KD Ti heterodimer and the in-variant 20 and 25 KD T3 molecules. Approximately 30,000-40,000 Ti and T3 molecules exist on the surface of human T lymphocytes. These glycoproteins are acquired and expressed during late thymic ontogeny, thus providing the structural basis for immunologic competence. The alpha and beta subunits of Ti bear no precursor-product relationship to one another and are encoded by separate germline V, D, J and C segments which rearrange during intrathymic differentiation to form an active gene set. Triggering of the T3-Ti receptor complex induces a rapid increase in free cytoplasmic Ca2+ and gives rise to specific antigen-induced proliferation through an autocrine pathway involving endogenous IL-2 production, release, and subsequent binding to IL-2 receptors. The implications of these findings for understanding of human T cell growth and its regulation in disease states are discussed.