Helicobacter pylori synthesizes and secretes a substance which co-chromatographs and is antigenically cross-reactive with the bacterial chemotactic peptide fMet-Leu-Phe. Using reverse phase and affinity chromatography this substance has now been purified. Carboxypeptidase Y microsequencing has verified that this material is fMet-Leu-Phe. The infiltration of polymorphonuclear leucocytes to sites of H. pylori infection may be a response to mucosal permeation of soluble, diffusable bioreactive substances such as fMet-Leu-Phe.