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Hag¢sh intestinal antimicrobial peptides (HFIAPs) are a family of polycationic peptides exhibiting potent, broadspectrum bactericidal activity. In an attempt to unravel the mechanism of action of HFIAPs, we have studied their interaction with model membranes. Synthetic HFIAPs selectively bound to liposomes mimicking bacterial membranes, and caused the release of vesicle-encapsulated £uorescent markers in a sizedependent manner. In planar lipid bilayer membranes, HFIAPs induced erratic current £uctuations and reduced membrane line tension according to a general theory for lipidic pores, suggesting that HFIAP pores contain lipid molecules. Consistent with this notion, lipid transbilayer redistribution accompanied HFIAP pore formation, and membrane monolayer curvature regulated HFIAP pore formation. Based on these studies, we propose that HFIAPs kill target cells, at least in part, by interacting with their plasma membrane to induce formation of lipid-containing pores. Such a membrane-permeabilizing function appears to be an evolutionarily conserved host-defense mechanism of antimicrobial peptides. ß 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.