The monoclonal antibody ER-MP42 specifically reacts with major histocompatibility complex (MHC) class I antigens of the mouse and, therefore, it is a valuable tool for studying cytotoxic T-cell interactions with class I positive antigen presenting cells. The primary immunological function of MHC molecules is to bind and present antigenic peptides on the surfaces of cells for recognition by the antigen-specific T cell receptors (TCRs) of lymphocytes. MHC class I molecules are expressed on the surfaces of most cells and are recognized by CD8-positive cytotoxic T-cells, an essential step for initiating the elimination of virally infected cells by T-cell-mediated lysis. MHC class I molecules are heterodimers composed of an alpha (44kD) and a beta (beta microglobulin, 11 kD) subunit. The first two structural domains of the alpha subunit associate to form the peptide-binding pocket.The monoclonal antibody ER-MP42 reacts with MHC class I, an antigen that is expressed by all somatic cells at varying levels. Lymphocytes are highly positive, whereas fibroblasts and neurons show only a low level of antigen expression. ER-MP42 recognizes murine MHC class I molecules on the surface of cells of the following haplotypes: H-2Fv, H-2Dd, H-2k,q,s. Weaker reactivity is found in mice of the following haplotypes: H-2p,r,w7,w22. MHC class I molecules of other haplotypes are not recognized by ER-MP42. There is no cross-reactivity with human MHC class I molecules.