Bacteria produce a heterogeneous mixture of neutrophil chemotactic agents in culture filtrates. Formylmethionyl peptides have been shown to comprise a significant portion of the chemotactic activity in bacterial culture filtrates; however, not all of the chemotactic agents in bacterial culture filtrates are formylated peptides. To examine whether nonformylated peptides derived from bacteria could act as chemotactic agents, we studied several nonformylated hepta- and octapeptide Enterococcus faecalis-derived sex pheromones, their modified derivatives, and their competitive inhibitors for activation of rat peritoneal neutrophils. Several of these peptides, in particular cAM373 and cPD1, proved to be potent chemotactic agents in submicromolar concentrations as well as inducers of lysosomal granule enzyme secretion. Moreover, the more biologically active peptides were able to compete with fMet-Leu-[3H]Phe for binding to the formyl peptide receptor. These studies demonstrate that the formylmethionyl moiety may be an absolute requirement only for the binding of di- and tripeptides to the formyl peptide receptor. Larger peptides that may have or that may allow for additional contact points between the peptide and receptor may require N-formylation only relatively. Indeed, by removing this structural restraint, the formyl peptide receptor may interact with an unlimited number of peptide fragments of both infectious and host origins to then modulate neutrophil responses to infection and inflammation.