In this study, we investigated the interaction of fengycin, a lipopeptide produced by Bacillus subtilis, with lipid monolayers using the Langmuir trough technique in combination with Brewster angle rnicroscopy. Thermodynamic analyses were performed to get further information about the mixing behavior and the molecular interactions between the two components. The effect of fengycin on the structural and morphological characteristics of DPPC monolayers, as a simple model of biological membranes, depends on the fengycin molar ratio. With a small proportion of fengycin (X-f less than or equal to 0.1), the compressibility of the monolayer is modified but the morphological characteristics of the DPPC are not significantly affected. At an intermediate molar ratio (0.1 < X-f less than or equal to 0.5), fengycin has a fluidizing effect on the DPPC monolayer by interacting partially with DPPC molecules. At higher molar ratio (X-f = 0.66), fengycin totally dissolves the ordered phase of the lipid. These results highlight the capacity of fengycin to perturb the DPPC organization and are discussed in relation to fengycin capacity to affect biological membranes. (C) 2004 Elsevier Inc. All rights reserved.