Strains of Enterococcus faecalis carrying the bacteriocinogenic plasmids pMB1 or pMB1.1 exhibit a clumping response to culture supernatants of different enterococcal strains. Antibodies raised against cells induced by a homologous pheromone recognize two surface proteins of 152 and 72.5 kDa (the second one is possibly the degradation product of the first), respectively. These antigens are very similar to those found in induced cells of E. faecalis OGIRF(pAM211) as shown by the cross-reaction of the immune sera obtained in this work. We propose that the 152-kDa protein corresponds to the aggregation substance coded by plasmids pMB1 and pMB1.1. Moreover, antibodies raised against induced cells are able to block the pheromone-induced clumping response. When the cells induced to form aggregates by pheromones were examined under a scanning electron microscope they showed a surface layer of hairlike structures.