Monoclonal antibodies allow for the detection of structures on the cell surface of human cytotoxic thymus-derived lymphocytes, which are involved in their function. Among these cell surface components, T8 is of particular interest, since it is required during the recognition of target cells by a subset of human cytotoxic T lymphocytes. The only other cell types on which T8 has been detected are functionally inert thymocytes. Here we demonstrate that T8 isolated from peripheral blood T lymphocytes is different from thymocyte T8. On peripheral blood T lymphocytes T8 was found in multimeric forms of a 34-kDa glycoprotein. These forms were also found on thymocytes, but in addition multimeric complexes of the 34-kDa T8 with a 46-kDa protein were detected. Preliminary chemical analyses showed that the 34-kDa and the 46-kDa forms differ in both their protein and carbohydrate structures. The 46-kDa structure was found to be distinct from the major thymocyte antigen T10. The interchain disulfide bridges between the 34-kDa T8 polypeptide are located in the 24-kDa CNBr fragment, which contains a hydrophobic region. Two (14 kDa and 20 kDa) of the three CNBr fragments of the 46-kDa T8 subunit were found to be involved in interchain disulfide bridging with the 34-kDa T8 species. We suggest that the switch from heteromultimers to homomultimers may occur concomitantly with the last step in T lymphocyte maturation.