A monoclonal IgG antibody was produced from a mouse immunized with an A11, A24; B27, B44 Epstein-Barr virus transformed B lymphoblastoid cell line. The antibody, A11.1M, by standard lymphocytotoxicity assay, reacts with all cells expressing HLA-A11 and -A24. Absorption studies with both A11+, A24- and A11-, A24+ platelets removed antibody reactivity against A11 and A24 lymphocytes. The shared antigenic determinant between A11 and A24, as defined by this antibody, A11.1M, represents a new "supertypic" determinant.