We investigated the interaction of the peptide melittin with differently sized vesicles consisting of various lipid compositions. This system was characterized by dynamic light scattering to estimate the size of vesicles. For SUV we obtained a radius of 12 nm, for LUV 53 nm. The pore forming process of melittin in vesicles was investigated by efflux of encapsulated fluorescent dyes at a self-quenching concentration. The influence of the following parameters on efflux and pore formation was estimated: lipid composition (POPC and DOPC), vesicle size (SUV and LUV) and size of the encapsulated dye (carboxyfluorescein and FITC-dextran). We found that under similar conditions vesicles of DOPC give always less leakage than vesicles of POPC independent of the fluorescent dye. For SUV and LUV we have obtained a different leakage behaviour at identical surface concentrations of melittin (if the same partition coefficient is assumed). From efflux measurements with different dyes we concluded that 6-20 molecules of melittin are necessary to form a pore. The possibility that not pore formation but fusion is the mechanism of melittin induced efflux was disproved by fusion experiments using a resonance energy transfer assay.