Fusafungine is a peptide antibiotic mixture composed of several enniatins and active against Gram-positive bacteria. Ionophoric properties of fusafungine have been studied in liposomes by measuring protoncation exchange by both fluorescence and 31P-nuclear magnetic resonance (NMR) and have been compared to those of its constituent enniatin peptides. Fusafungine, as well as enniatins, transport cations through a mobile carrier mechanism selective for K+ vs. Na+ and involving two antibiotic molecules. The transport efficiencies of the various enniatins appear to be related to their hydrophobicity, in agreement with a previously proposed "sandwich" transport model. The ionophoric properties of crude fusafungine may be involved in its antibiotic action and its local therapeutic properties.