Gramicidin S (GS) is a cyclic decapeptide of primary structure [cyclo-(Val-Orn-Leu-D-Phe-Pro)(2)] secreted by Bacillus brevis. It is a powerful antimicrobial agent with potent cidal action on a wide variety of Gram-negative and Gram-positive bacteria as well as on several pathogenic fungi. Unfortunately, however, GS is rather non-specific in its actions and also exhibits a high hemolytic activity, limiting its use as an antibiotic to topical applications. In a wide variety of environments, the GS molecule exists as a very stable amphiphilic antiparallel beta-sheet structure with a polar and a non-polar surface. Moreover, the large number of structure-activity studies of GS analogs which have been carried out indicate that this 'sidedness' structure is required for its antimicrobial action. In this review, we summarize both published and unpublished biophysical studies of the interactions of GS with lipid bilayer model and with biological membranes. In general, these studies show that GS partitions strongly into liquid-crystalline lipid bilayers in both model and biological membranes, and seems to be located primarily in the glycerol backbone region below the polar headgroups and above the hydrocarbon chains. The presence of GS appears to perturb lipid packing in liquid-crystalline bilayers and GS can induce the formation of inverted cubic phases at lower temperatures in lipids capable of forming such phases at higher temperature in the absence of peptide. The presence of GS at lower concentrations also increases the permeability of model and biological membranes and at higher concentrations causes membrane destabilization. There is good evidence from studies of the interaction of GS with bacterial cells that the destruction of the integrity of the lipid bilayer of the inner membrane is the primary mode of the antimicrobial action of this peptide. The considerable lipid specificity of GS for binding to and destabilization of lipid bilayer model membranes indicates that the design of GS analogs with an improved antimicrobial potency and a markedly decreased toxicity for eukaryotic cell plasma membranes should be possible.