A good correlation between 50% inhibitory concentration of hybrid liposomes (HL) composed of 90 mol% dimyristoylphosphatidylcholine and 10 mol% polyoxyethylene(n)dodecyl ether on the growth of human colon tumor (WiDr) cells, and membrane fluidity of HL was obtained. HL distinguished between WiDr and normal colon cells and then fused and accumulated into the membranes of WiDr cells leading to apoptosis.