Ribosome-inactivating proteins are enzymes of plant origin which de-adenilate the major ribosomal RNA, making it unable to bind the elongation factor and thus arresting protein synthesis. Recently the N-glycosidase activity of these enzymes has been extended also to deoxyribonucleotides substrates. In the present study we report the successful entrapment of the type 1 ribosome-inactivating protein saporin, covalently labelled with fluorescein isothiocyanate (FITC) into L-alpha lecitin/cholesterol liposomes and describe its delivery to human melanoma cells in vitro. The fluorescein reacted toxin maintained its enzymatic activity, although to a reduced extent; its interaction with liposomes resulted in the entry of the protein through the lipid bilayers. The resulting vesicles are carriers that can deliver the toxin inside cells; as a consequence the cytotoxic effects of the encapsulated enzyme were evident at a concentration two order of magnitude lower than that of the native one. In particular the nuclear damage, as revealed by micronuclei formation, was evident within 44 hr. The intracellular dynamics of the enzyme, as analyzed by confocal microscopy, point to an endocytic pathway of vesicles entry.