Immunostimulation effects of hybrid liposomes (HL25) composed of 90 mol% l-alpha-dimyristoylphosphatidylcholine (DMPC) and 10 mol% polyoxyethylene(25)dodecyl ether (C12(EO)25) on the normal human peripheral blood mononuclear cells (hPBMCs) were investigated in vitro. The concentrations of cytokines (IFN-gamma, IL-12) were significantly increased in the presence of HL25 ([DMPC] = 100 microM, [C12(EO)25] = 10 microM) and the maximum values attained were 13-14 times higher compared with those of control, though the viability and proliferation of hPBMCs were decreased under the same conditions. Such a remarkably high cytokine-production induced by liposomes without any stimulators was obtained for the first time. Furthermore, fluorescence microscopic and flow cytometric analyses indicated that the HL25 could be incorporated into hPBMCs by the membrane fusion and/or the endocytosis. These results suggest that HL25 should induce the effective immunopotentiation by stimulating the cytokine productivity of hPBMCs.