Production of bacteriocin Bc-48 by Enterococcus faecalis S-48 is encoded by the conjugative plasmid pMB1, which is approximately 90 kb and also responds to sex pheromones of E. faecalis OG1X. Mutants harboring deleted forms of this plasmid (pMB1-del, 75 kb) have lost both the phenotype Bc-48 (production and immunity) and the clumping response. The conjugal transfer of pMB1 to E. faecalis OG1X results in the acquisition by this strain of both bacteriocin production and immunity and also the clumping response. In the transconjugants isolated, the bacteriocinogenic trait is associated with a smaller plasmid (52 kb), which we call pMB1-1. The relationship among plasmids pMB1, pMB1-del, and pMB1-1 has been demonstrated by DNA hybridization. Plasmid pMB1-1 has been transferred with high frequency to E. faecalis mutants cured of Bc-48 production (carrying pMB1-del), conferring to them the Bc-48 trait and clumping response. In the transconjugants from a second mating, pMB1-1 and pMB1-del coexist without appreciable segregation.