The interactions between charged unilamellar vesicles of different size prepared from mixtures of egg lecithin (EPC(1)), cholesterol, and a charged component, either stearylamine or dicetyl phosphate, were investigated. Several techniques, i.e. fluorescence spectroscopy, Cs-133-NMR, light scattering measurements, and electron and optical microscopies in dilute aqueous solutions, provided evidence that vesicles bearing opposite charges interact via contact followed by lipid exchange; the progressive charge neutralization is shown by Cs-133-NMR to occur without fusion of vesicle internal pools. Vesicle size determines as diverse features as contact duration, extent of lipid exchange, and final distribution of surface charge among the vesicle mixture.