The effect of lipid composition on the sensitivity of liposomes (multilamellar liposomes) and sonicated liposomes, which were composed mostly of single-compartment liposomes, toward Triton X-100, a detergent, was examined. Monomeric molecules of Triton X-100 seem to penetrate into liposomal bilayers, since Triton X-100 could modify the permeability of liposomes at concentrations below its critical micellar concentration. Cholesterol incorporation into egg lecithin liposomes had a negligible effect on the sensitivity to Triton X-100. On the other hand, cholesterol incorporation drastically reduced the Triton-induced permeability change of multi-lamellar liposomes prepared with dipalmitoyl and dimyristoyl phosphatidylcholines. Liposomes prepared with dipalmitoyl phosphatidylcholine or dimyristolyl phosphatidylcholine showed a strong resistance to the action of the detergent when prepared with 50 mol% of cholesterol, releasing at most 10% of trapped glucose. Ultrasonicated liposomes of dipalmitoyl phosphatidylcholine were, however, still sensitive to Triton X-100, releasing almost 100% of the trapped glucose even when 50 mol% of cholesterol was incorporated. This fact indicates that only the outermost bilayers in multilamellar liposomes composed of dipalmitoyl phosphatidylcholine and cholesterol may be sensitive to the Triton X-100. In cases of multilamellar liposomes with less than 33 mol% of cholesterol, sensitivities of liposomes to the detergent decreased in the following order; dipalmitoyl phosphatidylcholine, dimyristoyl phosphatidylcholine, egg lecithin and rat liver phosphatidylcholine liposomes. The same order of sensitivity to Triton was also observed in the system of sonicated liposomes. Membranes in the gel state are most sensitive to the detergent, followed by membranes around the phase transition. Egg and rat liver phosphatidycholine membranes, both of which are liquid-crystalline, were the least sensitive to the detergent.