A chemotactic tetrapeptide from culture fluids of Staphylococcus aureus was purified to homogeneity by reverse-phase high-pressure liquid chromatography. The peptide comprises equimolar methionine, leucine, isoleucine, and phenylalanine. It inhibited binding of fluoresceinated fMet-Leu-Phe-Lys to human monocytes, which showed that it interacted with the formyl-methionyl peptide receptor and suggested that it was a formyl-methionyl peptide. Based on a comparison of dose-response curves for inhibition of fluoresceinated fMet-Leu-Phe-Lys binding, the relative affinity of the peptide for the receptor was comparable to that of fMet-Leu-Phe-Lys. At optimal concentrations, chemotactic efficacy (percentage of monocytes migrating to the attractant) was 53 +/- 4%, in contrast to 36 +/- 3% for the reference attractant fMet-Leu-Phe. Since approximately 60% of human monocytes have formyl-peptide receptors, the bacterial peptide is capable of attracting all receptor-bearing monocytes.