In order to get more standardized reagents for blood group typing and to get more information on the chemical nature of blood group antigens we have immunized Balb/c mice with whole red blood cells and with sialoglycoprotein fraction from group OMMss cells. As fusion partners mouse myeloma cell lines P3-NS1/1-Ag 4-1 and X63-Ag8.653 were used. Several monoclonal antibodies were obtained, which showed specificity for the M/N antigens. Because of the relationship between M and N antigens this system is a very good example for studying the influence of different parameters on the reactivity of monoclonal antibodies. Each monoclonal antibody behaves like an individuum and for each antibody the dependence of the agglutination reaction of temperature, pH, buffer for dilution and medium, in which the reaction takes place, has to be investigated. Otherwise the results are not comparable and unspecific reactions might occur. By standardizing these parameters specific reactions were obtained and the antibodies might be used as routine reagents.