The strain Enterococcus faecalis S-48 carries two large conjugative plasmids (pMB1 and pMB2) encoding for antagonistic substances. The pheromone response of these two plasmids was established by purifying the corresponding sex pheromones, using conventional reversed-phase columns. Plasmid pMB1 responds to pheromone cCF10. Plasmid pMB2 responds to a pheromone with an amino acid sequence identical to that of cPD1 (Phe-Leu-Val-Met-Phe-Leu-Ser-Gly). The two pheromone-responding plasmids coexist in a stable fashion in the wild-type strain E. faecalis S-48.