Monoclonal antibodies allow for the detection of antigens which are specific for human thymus-derived lymphocytes. Among these antigens, the T3 complex is of particular interest since it is involved in several T cell functions. The main target antigen of the anti-T3 reagents is borne by a 20-kDa glycoprotein. In addition, glycoproteins of 25-28, 37, and 44 kDa are found in anti-T3 immunoprecipitates derived from surface-labeled cells. The four antigens appeared to be strongly associated with each other in detergent-containing solutions. Comparative studies of the four proteins, facilitated by the use of endo-beta-N-acetylglycosaminidase F, revealed that their polypeptide backbones have different molecular weights and pI values. Moreover, peptide maps of the 20-kDa T3 and the 25-28-kDa T3 were quite different. Metabolic labeling experiments suggested that the 25-28-kDa protein might become associated with the 20-kDa T3 antigen during biosynthesis. The 37-kDa and 44-kDa proteins could not, however, be detected and, therefore, might become associated with the 20-kDa T3 on the cell surface. Evidence has been found for the existence of a fifth member of the T3 complex, namely an unglycosylated 20-kDa T3 species.