Melittin, the principal toxic component of bee venom, is a cationic, amphipathic, linear peptide composed of 26 amino acids, which exhibits unique structural and biological characteristics. It has high antimicrobial activity but also has the very detrimental property of killing eucaryotic cells, as illustrated by the lysis of sheep red cells. Several attempts have been made through synthesis of replacement analogs to advance the molecular understanding of the cause of these effects. We have now synthesized retro melittin, an amphipathic alpha-helical analog with reversal of sequence and therefore of the positions of charged and apolar residues, notably, the cluster of basic residues Lys(21)-Arg-Lys-Arg(24) near the C-terminus which is now located at positions 3-6 near the amino terminus. This peptide retained high antimicrobial activity against a range of test bacteria, but lost much of its hemolytic properties. Modification of the N-terminal positive charge by acetylation did not further alter the antibacterial activity or red cell lysis. The synthetic retroenantio melittin (all-D isomer) and its acetylated derivative both retained full antibacterial activity, but with complete elimination of the hemolytic effect. Therefore, the two effects of melittin have been separated. Melittin and these analogs promote electrical conductivity in lipid bilayers. Circular dichroism measurements showed that all of these peptides-normal, enantio, retro, retroenantio, and their acetylated derivatives-were 80-100% helical in 12-20% hexafluoro-2-propanol, a structure inducing solvent, and they are thought to be helical in lipid bilayers and bacterial membranes, Nonhelical analogs are inactive. It is believed that the helix dipole plays a major part in orienting the peptides in membranes. Active sequences are not unique, but sequence plays a role in peptide conformation and activity. Chirality has virtually no role in the antibacterial activity of normal and retro melittin analogs, which leads to the conclusion that these peptides do not function via a receptor or by enzymatic processing, but by self-aggregation and formation of ion-conducting pores.