The “classical” MHC Class I molecules are histocompatibility antigens encoded by the H-2 gene complex and consist of heterodimers of highly polymorphic ? chains noncovalently associated with the invariant ?2-microglobulin. These antigens are expressed on most nucleated cells but expression varies on different cell types. MHC Class I molecules present endogenously synthesized peptides to CD8+ T lymphocytes, which are usually cytotoxic T cells. MHC Class I antigens expressed on thymic epithelial cells regulate the positive and negative selection of CD8+ T cells during T cell ontogeny. The monoclonal antibody 34-1-2S binds to a common determinant in the ?3 domains of H-2Kd and H-2Dd in the presence or absence of ?2 microglobulin. It cross reacts with the ?3 domain of H-2Kb.