Surfactant-like membranes containing the 21-residue peptide KLLLL-KLLLLKLLLLKLLLLK (KL4 ), have been clinically tested as a therapeu-tic agent for respiratory distress syndrome in premature infants. The aims of this study were to investigate the interactions between the KL4 peptide and lipid bilayers, and the role of both the lipid composition and KL4 structure on the surface adsorption activity of KL4 -containing membranes.