We report on the reversible association of anionic liposomes induced by an antimicrobial peptide (LAH4). The process has been characterized for mixed membranes of POPC and POPS at molar ratios of 1:1, 3:1, and 9:1. Although the vesicles remain in suspension in the presence of excess amounts of peptide, the addition of more lipids results in surface charge neutralization, aggregation of the liposomes, and formation of micrometer-sized structures that coexist in equilibrium with vesicles in suspension. At low ratios of anionic lipids, vesicle aggregation is a reversible process, and vesicle disassembly is observed upon inversion of the surface charge by further supplementation with anionic vesicles. In contrast, a different process, membrane fusion, occurs in the presence of high phosphatidylserine concentrations. Upon binding to membranes containing low POPS concentrations, the peptide adopts an in-plane alpha-helical structure, a secondary structure that is conserved during vesicle association and dissociation. Our finding that peptides are essential for vesicle aggregation contributes to a better understanding of the activity of antimicrobial peptides, and suggests an additional layer of complexity in membrane-protein lipid interactions.