PR-39 is a 4719-Da peptide isolated from pig intestine and belonging to the recently discovered family of Pro+Arg-rich antibacterial peptides. PR-39 does not lyse Escherichia coli, instead the lethal action is probably linked to the termination of DNA and protein synthesis [Boman, H. G., Agerberth, B. & Boman, A. (1993) Infect. Immun. 61, 2978-2984]. Circular dichroism and Fourier-transform infrared spectroscopy have been used to investigate the secondary structure of PR-39 in the absence or presence of lipids. According to the circular dichroic data, this structure is not altered upon incubation of PR-39 with negatively charged vesicles, although the infrared spectra suggest that the hydrogen bond pattern is modified upon the peptide-lipid interaction. This is detected by a shift in the maximum wavelength of absorption of PR-39 from 1636 cm-1 in the absence of lipids to 1645 cm-1 in the presence of lipids. We have further addressed the question of the possible mechanism of interaction of PR-39 with model membranes (liposomes and planar lipid bilayers) whose lipid compositions mimick that of the E. coli inner membrane. PR-39 induced a calcein release from large unilamellar vesicles, which is dependent upon the peptide concentration and upon the presence of negatively charged lipid (glycerophosphoglycerol) in the membrane. The binding study of PR-39 to dioleoylglycerophosphoglycerol vesicles suggests that nearly 100% of the added peptide is membrane-bound. Addition of PR-39 to a planar lipid bilayer induced a linear increase in the current but no channel formation was observed since no discrete steps of conductance occurred.