Indolicidin, a tryptophane-rich antimicrobial peptide, was used to investigate the interactions with a zwitterionic phosphatidylcholine as a model membrane system. In situ atomic force microscopy in liquid medium and phosphatidylcholine supported planar bilayers enabled the study of the interactions between indolicidin and the lipid membrane in real time. It was evident that indolicidin induced a continuous shrinking and thinning of the supported planar bilayers. The effect of indolicidin was dependent upon the composition and physical properties of the membrane bilayer. The interaction of indolicidine with the membrane could be best and most pronounced seen and studied at the boundary of the gel-fluid-domains. Dye leakage experiments with phosphatidylcholine vesicles encapsulated with calcein revealed that indolicidin induced fluidisation of the lipid membrane leading to dye release. The present study indicates that the mode of action for indolicidin can be best described by a stepwise interaction of the peptide with the membrane. Formation of pores however can not be supported on the basis of our experiments.