The agr quorum-sensing system is responsible for the regulation of several virulence factors in staphylococci, with an extracellular pheromone peptide as signalling molecule. By monitoring the biological activity of synthetic peptides, it could be demonstrated that the pheromone of the agr system in Staphylococcus epidermidis is an octapeptide containing a thiolester linkage between the central cysteine and the C-terminal carboxyl group. The peptide was active at nanomolar concentrations. The N-terminus of the peptide pheromone, which is encoded as part of a protein precursor, proved to be crucial for biological activity.