The Streptococcus faecalis sex pheromone cPD1, which induces a mating response in cells harboring the conjugative plasmid pPD1, has been isolated and its structure determined. It was found to have a molecular weight of 912, and its amino acid sequence was H-Phe-Leu-Val-Met-Phe-Leu-Ser-Gly-OH. A synthetic octapeptide showed the same biological activity and chromatographic behavior as the isolated cPD1. Pheromone activity was detectable at a concentration of approximately 4 X 10(-11)M.