The interactions of the antibiotic polymyxin BI and its enzymatic cleavage product polymyxin B~ nonapeptide with phosphatidic acid monolayers and with bilayer membranes were investigated. Temperature-dependent pressure-arca analysis of the monolayer reveals a linear increase of the lipid mean molecular area in the liquid condensed state for polymyxin concentrations between 10"* and 4 × 10-' M. Depending on the surface pressure, the area increase amounts to 30--70 ~2. A finear dependence was also observed in the liquid expanded state but saturation is reached already at 10-' M polymyxin. The adsorption of polymyxin to phosphatidic acid bilayers is also linear and of a Langmuir type. Saturation is reached at a 1 : 4 polymyxin/lipid molar ratio. Polymyxin induces a phase separation in phosphatidic acid monolayers which was concluded from the thermotropic phase transition curves. In agreement with earlier bilayer experiments a second lowered phase transition appears in the presence of polymyxin. These fluidized domains again exhibit a linear polymyxin uptake comparable to the one of the liquid expanded monolayer at a temperature, where the undisturbed lipid is still in the condensed state. Polymyxin nonapeptide also causes an expansion of phosphatidic acid monolayers but only by maximally 10 ,~2. The thermotropic phase transition of the monolayer is reduced and considerably broadened by the nonapeptide. In phosphatidic acid bilayers we observed a decrease of the lipid phase transition temperature by 24°C. The lateral chain packing is considerably disturbed by the peptide part of polymyxin. The branched fatty acid tail may only serve to anchor the peptide within the matrix but does not contribute considerably to the formation of less ordered subcells within the membranes. For polymyxin B, nonapeptide, the binding curve of peptide to bilayers is linear but saturation was not reached.