beta -Peptides are a class of polyamides that have been demonstrated to adopt a variety of helical conformations. Recently, a series of amphiphilic L+2 helical beta -peptides were designed, which were intended to mimic the overall physicochemical properties of a class of membrane-active antimicrobial peptides, including magainin and cecropin. Although these peptides showed potent antimicrobial activity, they also showed significant activity against human erythrocytes. Operating under the assumption that their lack of specificity arose from excessive hydrophobicity, two additional beta -peptides H-(beta (3)-HAla-beta (3)-HLys-beta (3)-HVal)(n)-NH2 (n = 4, 5) were designed and synthesized. Both have high antimicrobial activities, but very low hemolytic potencies. The peptides bind in an L+2 conformation to phospholipid vesicles, inducing leakage of entrapped small molecules. The peptides have a low affinity for membranes consisting of neutral phosphatidylcholine lipids, but bind avidly to vesicles containing 10 mol % of acidic phosphatidylserine lipids. Differences in vesicle leakage kinetics for the two peptides suggest that chain length could affect their mechanisms of disrupting cell membranes. Thus, insights gained from the study of variants of natural a-peptides have provided a useful Guide for the design of nonnatural antimicrobial beta -peptides.