Megasphaera cerevisiaeis a Gram-negative obligate anaerobe that causes turbidity and off-flavour and aroma in beer. Seven isolates ofM. cerevisiaewere obtained worldwide, and their extractable surface antigens were focused upon to determine if there is more than one serogroup of this bacterium. Sodium dodecyl sulphate polyacrylamide gel electrophoresis of ethylenediaminetetraacetic acid (EDTA) bacterial extracts revealed a predominant protein with apparent molecular weights of 46 000, 45 000, and 43 000 for three, two, and two isolates, respectively. When mouse antiserum generated against any of the EDTA extracts was reacted with denatured bacterial proteins in immunoblots, all bacterial isolates exhibited extensive cross-reactivity involving three antigens, one being the major EDTA-extractable protein. In contrast, when the sera were tested for surface reactivity with intact bacteria, three cross-reactivity groups were observed, with the groups individually comprised of bacteria having the same size major EDTA-extractable surface protein. When BALB/c mice immunized with a bacterium from each of the three serogroups were used for monoclonal antibody (Mab) hybridoma production, bacterial surface-reactive Mabs were obtained whose reactivities parallel the three polyclonal antibody-defined serogroups. Through combining these surface-reactive Mabs, it will be possible to rapidly detect and identify beer contamination byM. cerevisiaebelonging to any serogroup.Key words: beer spoilage bacteria,Megasphaera cerevisiae, monoclonal antibodies, surface proteins, serogroups.