We have studied the interaction ofb-17, a potent synthetic antimicrobialb-peptide, with phospholipids. We find that unlikeother antimicrobial peptides suchasmagainin II,b-17 facilitates the formation of nonbilayer phases, indicating that the peptide promotes negative curvature. Studies of liposomal leakage also indicate a different mode of mem-brane interaction relative to magainin II, but both leakage andmembrane binding showthatb-17, likemagainin II, has strongaffinity formembranes containinganionic lipids