The emergence of antibiotic resistant strains is an impetus for the research towards the development of novel antimicrobial agents. Especially attractive are lead compounds that function in a different way than the mode of action of commonly used antibiotics. The cationic antimicrobial cyclic peptide gramicidin S (S = Sovjet) is such a lead compound as it disrupts the barrier function of biological membranes. It is a cyclic decapeptide with the primary sequence c-(P-V-O-L-dF)₂ that adopts a C₂-symmetric amphiphilic anti-parallel cyclic hairpin secondary structure. Gramicidin S primarily kills Gram positive bacteria, but is also toxic to other cells, including human red blood cells, as its disruption of membranes is non-specific. Due to this property, gramicidin S is only applied topically. Insight in its mechanism of action may lead to the development of derivatives with an improved biological profile.