SDS extracts of whole bacteria, representing five species and 15 serovars of Listeria, were analysed by SDS-PAGE and by immunoblotting with serum directed against whole formalin-treated L. monocytogenes. Profiles of L. monocytogenes were very different from those of other species of Listeria (i.e. L. innocua,L.welshimeri, L. seeligeri and L. ivanovii). This low degree of similarity between species was found even in the case of common serovars. Within the species L. monocytogenes, protein patterns were characterized, on the one hand, by a high degree of homogeneity between all strains of the same serovar and, on the other hand, by large differences between serovars, especially between sv. 1/2 and 4b. Thus we have identified major, surface-located protein antigens, specific for L. monocytogenes, either common to all serovars (64 and 68 kDa) or characteristic of certain serovars: 98 kDa for sv. 1/2 and 3; 76 and 78 kDa for sv. 4b, 4d and 4e; and 80 and 100 kDa for sv. 4a and 4c. Moreover, some of these bands (68 and 98 kDa) might be related to virulence, since differences were noticed between the profiles of haemolytic L. monocytogenes vs. 1/2a differing only in their virulence for immunocompromised mice. All these results confirmed, for the first time, the classification of Listeria obtained previously by genomic studies. They should help in the identification of new virulence factors and the development of easier and more specific methods of detection and identification.