Deep rough mutant lipopolysaccharide (ReLPS) dissolved in aqueous solution spontaneously forms supramolecular structures which mainly consist of vesicles, Addition of Polymyxin B (PmB) to these ReLPS vesicles influence the shape of these structures as demonstrated here by electronmicroscopy and dynamic light scattering techniques, The main phase transition of the ReLPS is found at 21.3 +/- 0.1 degrees C for ReLPS from Escherichia coli and at 24.0 +/- 0.5 degrees C for ReLPS from Salmonella minnesota by differential scanning calorimetry (DSC), Using isothermal differential titration calorimetry (ITC), the thermodynamic behavior of the interaction of PmB with ReLPS vesicles has been studied. The stoichiometric ratio for the binding of PmB to ReLPS was found to lie between 0.6 and 1, as determined from ITC and monolayer experiments, No phase transition was observed for ReLPS monolayers saturated with PmB, The results indicate specific interaction of PmB with ReLPS. We propose a two-step mechanism for this interaction, which involves electrostatic attraction between charged parts of the molecules and, in the second step, hydrophobic interactions between the nonpolar parts of both compounds.