Cloned populations of T cells have been used to derive clone-specific monoclonal antibodies. These antibodies influence the specific immunological functions of the cloned T cells, suggesting that these antibodies react with the T cell receptor for antigen. Immunoprecipitates have been obtained with some of these antibodies, and the characteristics of the polypeptides from different cells are surprising similar. At present, little is known about this molecular complex other than that the two polypeptides are disulfide-linked and have apparent molecular weights of about 45,000 daltons and 43,000 daltons. This molecular complex, at least in human T cells, is associated with, but not covalently linked with, another molecular complex (Leu4/T3). However, modern techniques of cellular and molecular immunology should provide information very soon about the actual role of these molecules in T cell recognition of antigen. Indeed, cDNA clones that may encode genes for one of the peptide chains have been obtained.