Polycationic peptides with seven or more arginine residues are capable of crossing cell membranes and can be used as vectors to carry covalently attached moieties and even nanoparticles into cells. On the other hand, cationic amphiphilic peptides can act as antimicrobial agents, usually by disrupting bacterial membranes. We compared the ability to cause disruption of liposome membranes of two peptides: a polycationic vector peptide (Naft-ARRRPRRRRAA-NH₂ where Naft is 1,8-naphthaleneimide) and a typical antimicrobial peptide, DDK (GLWSKIKAAGKEAAKAAAKAAGKAALNAVS) that is present in the skin secretion of the frog Phyllomedusa distincta. The peptides were synthesized by solid-phase, purified by reverse phase chromatography and identified by ESI-MS. DDK at 10^-6 mol L^-1 caused 93 % leakage of fluorescein from egg lecithin vesicles when compared with leakage caused by Triton X (100%). In contradistinction negligible leakage was observed when the vesicles were incubated in presence of the vector peptide, in agreement with data published previously for the polycationic peptides penetratin and heptaarginyl-tryptophan (R₇W). DDK capacity to cause fluorescein leakage was evaluated at increasing concentrations of cholesterol - for lecithin vesicles containing 15 % cholesterol the extent of leakage was 54 %. In the same conditions, the vector peptide still did not cause significant leakage of fluorescein from the lecithin vesicles. Thus, cholesterol reduces the ability of DDK to disrupt lecithin vesicles but has no influence on the considered vector peptide. The kinetic constants for fluorescein leakage for DDK are also being measured at varied cholesterol percentages (up to 15 %) in lecithin vesicles and the effects of the two peptides upon liposome vesicles will be evaluated by surface plasmon resonance.