Human major histocompatibility complex (MHC) class I antigen-epitope peptide complex associated with β2-microglobulin are expressed by all human nucleated cells. It plays an important role in cell-mediated immune responses. Abnormalities in MHC class I antigen surface expression are frequently found in malignancies and infectious diseases. They are often associated with reduced recognition by MHC class I antigen-restricted, tumor or virus-associated antigen-specific cytotoxic T lymphocytes and disease progression. The EMR8-5.1 monoclonal antibody reacts with a non-polymorphic epitope of human MHC class I antigens, HLA-A, -B, and -C. This antibody is suitable for study of MHC class I down-regulation using routine formalin-fixed paraffin-embedded sections by immunohistochemistry.