The interaction of amphiphilic lipopolysaccharides (LPS, endotoxin) from Gram-negative bacteria with the polycationic decapeptide polymyxin B (PMB) and its non-apeptide PMBN was studied mainly by applying isothermal titration calorimetry (ITC). The LPS investigated comprise various mutant and wild-type forms from Salmonella minnesota, differing in the length of their sugar chain, and some wild-type LPS from other strains. It can be shown that three effects occur after binding of the peptides to LPS, an electrostatic attraction of the positive charges of the polymyxins to the negative charges in particular in or near the lipid backbone of LPS (termed lipid A) leading to a calorimetric exotherm, a reorientation of the LPS aggregate structure, and a fluidization of the LPS acyl chains which are connected with endothermic processes. The superposition of these three effects. leads to sometimes very complex titration curves, apparently due to the presence of further negative charges distal to lipid A. The results from the ITC experiments allow the determination of the binding stoichiometry-which is important for the adjustment of dosages in therapeutical applications-lying at a molar ratio [PMB]: [LPS] of 1: 1.5 for LPS with shorter sugar chains and 1:2 to 1:3 for those with longer sugar chains (wild-types).