Some series of liposomal formulations are described, loaded with three oligopeptides: TP5, TP4, and TP3 derived from the thymic hormone thymopoietin. Liposomes, both multilamellar (MLV) and stable plurilamellar vesicles (SPLV) were prepared with and without cholesterol, and contained stearylamine or dipalmitoylphosphatidylserine to induce a positive or negative charge, respectively. The encapsulation efficiency and the rate of release of the entrapped drugs, after incubation at 37 degrees C, were evaluated and results were discussed as a function of liposome type, charge and composition. In vitro evaluation of cytotoxic activity of the three peptides on K562 and IM9 human tumor lines showed in some cases (TP3 and TP5-loaded positively charged MLV; IM9 cells) a slight increase of the activity of peptides. However, a relevant intrinsic toxicity of pure phospholipids (empty liposomes) on cell cultures was observed, which altered the evaluation of drug-loaded formulations activity.