Interactions of the cationic amphipathic peptide KLALKLALKALKAALKLA-NH(2) (KLAL) and its double D-amino acid replacement analogues l(11)k(12)-KLAL and k(9)a(10)-KLAL with lipid monolayers of anionic POPG, zwitterionic POPC and mixtures thereof at the air/water interface were investigated by infrared reflection- absorption spectroscopy (IRRAS). At high surface pressure (>30 mN m(-1)) all peptides incorporated into lipid monolayers containing at least 25 % anionic POPG, and adopted an alpha-helical conformation. Creation of free surface by expansion of the monolayers resulted in an additional adsorption of peptides from the subphase, but now in a beta-sheet conformation; this led to the coexistence of peptides in two distinctly different conformations within the lipid monolayer. The beta-sheets bound to the free surface could be squeezed out of the film by compressing the film to low surface areas, whereas the alpha-helices remained bound to the lipids until the film collapsed. When bound to the lipid monolayer, the helical axis of the peptides is oriented almost parallel to the surface of the monolayer.