In Enterococcus faecalis, lateral transfer of conjugative plasmids that encode antibiotic resistance and virulence determinants can be induced by peptide sex pheromones. The tetracycline-resistance plasmid pCF10 represents a paradigm for illustrating important conserved features of a large family of pheromone-responsive enterococcal plasmids. The pheromone is released into the growth medium by plasmid-free recipient cells and sensed by plasmid-containing donors. The activity of the pheromone is antagonized by a plasmid-encoded inhibitor peptide that prevents conjugation in the absence of an inducing signal and is also required to return the system to the ground state following an induction cycle. The pheromone response involves multiple transcriptional and posttranscriptional mechanisms as well as bi-stable biological switch behavior. Multiple layers of regulation are essential for proper function, and evolution of this tight control system may have been favored by reduction of the fitness cost of plasmid maintenance to the host cell.