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 composed of a clonotypic 90-kDa Ti heterodimer and the invariant 20- and 25-kDa 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 Ti alpha and Ti beta subunits bear no precursor-to-product relationship and are encoded by separate germ line 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.