(1) The interaction of tyrocidine with different lipids is studied in model membranes and the results are compared to the gramicinid-lipid interaction. (2) The tyrocidine-dielaidoylphosphatidylethanolamine interaction gives rise to a population of phospholipids with a lower gel to liquid-crystalline transition temperature and to an abolition of the bilayer to HII phase transition, resulting in a macroscopic organization with dynamic and structural properties different from those of the pure lipid. (3) Tyrocidine has a strong fluidizing effect on the acyl chains of phosphatidylcholines, manifested by a decrease in enthalpy of the main thermotropic transition. (4) No evidence of a gramicidin A'-like lipid-structure modulating activity was found. However, tyrocidine inhibits the formation by gramicidin of an HII phase in dioleoylphosphatidylcholine model membranes. Instead, a cubic type of lipid organization is observed. (5) Tyrocidine greatly perturbs the barrier properties of dioleoylphosphatidylcholine model membrane. (6) Gramicidin A' reverses the effect of tyrocidine on membrane permeability by forming a complex in the model membrane with an apparent 1:1 stoichiometry. (7) The results suggest that both peptide antibiotics, which are produced by Bacillus brevis ATC 8185 prior to sporulation, show antagonism in their effect on membrane structure similar to their effect on superhelical DNA (Bogh, A. and Ristow, H. (1986) Eur. J. Biochem. 160, 587-591. The possible underlying basic mechanism is indicated.