The inhibitory effects of hybrid liposomes on the growth of human breast tumor (MDA-MB-453) cells in vitro were examined. The 50% inhibitory concentration of hybrid liposomes composed of 95 mol% dimyristoylphosphatidylcholine (DMPC) and 5 mol% polyoxyethylenedodecyl ether (C-12(EO)(23)) was remarkably reduced compared with that of DMPC liposome and C-12(EO)(23) micelle. It was noteworthy that the induction of apoptosis by hybrid liposomes without antitumor drugs was obtained for human breast tumor cells.