A synthetic cecropin A(1-8)-melittin(1-18) hybrid peptide, with antimalarial and antibacterial properties, promotes leakage of aqueous contents of phospholipid vesicles, as determined by measuring the induced release of vesicle-entrapped fluorescence probes. The release of vesicle contents corresponds to an all-or-none mechanism. High molecular weight entrapped solutes (fluorescence-labeled dextrans, 20 and 4 kDa molecular mass) are also released by the peptide. This fact and the high peptide stoichiometry required for the release of vesicle contents suggest a detergent-like disruption of the bilayer. The leakage process is not related to any membrane event requiring lipid-mixing between bilayers. The peptide destabilizes both negatively and neutrally charged phospholipid vesicles. The thermal variation of the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene-labeled vesicles is modified by the peptide. Circular dichroism and tryptophan fluorescence emission spectra reveal conformational changes in the peptide molecule upon interaction with the lipid vesicles. These changes are consistent with an increased alpha-helical content and a less polar environment for the single tryptophan residue of the peptide. The leakage induced in phosphatidylserine vesicles is a faster process than in phosphatidylcholine vesicles, while the peptide is more effective in releasing the contents of the latter type of vesicles. This suggests that acidic phospholipids may modulate the effect of the peptide on membranes.