The TCR?d heterodimer of the CD3/T cell receptor complex is clonotypic and consists of Ig-like variable and constant domains. It is expressed during thymopoiesis and on the small subpopulation of ?d TCR-expressing T lymphocytes in the periphery. The majority of T cells present in some epithelial tissues are ?d+ and have limited receptor diversity. The TCR?d heterodimer recognizes peptide antigen bound to MHC antigens and subsequent signal transduction mediated by the invariant chains leads to T cell activation. The monoclonal antibody UC7-13D5 is specific for the ?d heterodimer and plate-bound UC7-13D5 activates ?d TCR-bearing cells. This antibody does not react with aß TCR-expressing T cells.