Defensins, a family of cationic peptides isolated from mammalian granulocytes and believed to permeabilize membranes, were tested for their ability to cause fusion and lysis of liposomes. Unlike alpha-helical peptides whose lytic effects have been extensively studied, the defensins consist primarily of beta-sheet. Defensins fuse and lyse negatively charged liposomes but display reduced activity with neutral liposomes. These and other experiments suggest that fusion and lysis is mediated primarily by electrostatic forces and to a lesser extent, by hydrophobic interactions. Circular dichroism and fluorescence spectroscopy of native defensins indicate that the amphiphilic beta-sheet structure is maintained throughout the fusion process. Taken together, these results support the idea that protein-mediated membrane fusion depends not only on hydrophobic and electrostatic forces but also on the spatial arrangement of the amino acid residues to form a three-dimensional amphiphilic structure, which promotes the efficient mixing of the lipids between membranes. A molecular model for membrane fusion by defensins is presented, which takes into account the contributions of electrostatic forces, hydrophobic interactions, and structural amphiphilicity.