The tetracycline resistance plasmid pCF10 represents a class of unique mobile genetic elements of the bacterial genus Enterococcus, whose conjugative transfer functions are inducible by peptide sex pheromones excreted by potential recipient cells. These plasmids play a significant role in the dissemination of virulence and antibiotic resistance genes among the enterococci, which have become major nosocomial pathogens. Pheromone response by plasmid-carrying donor cells involves specific import of the peptide signal molecule, and subsequent interaction of the signal with one or more intracellular regulatory gene products. The pheromones are chromosomally encoded hydrophobic octa- or hepta-peptides, and different families of homologous plasmids encode the ability to respond to each pheromone. Among the four pheromone-responsive plasmids that have been characterized in some detail, there is considerable conservation in the genes encoding pheromone sensing and regulatory functions, and the peptides themselves show considerable similarity. In spite of this, there is extremely high specificity of response to each peptide, with virtually no "cross-induction" of transfer of non-cognate pheromone plasmids by the pheromones. This communication reviews the evidence for this specificity and discusses current molecular and genetic approaches to defining the basis for specificity.