The properties of multibilayered liposomes formed from mixtures of sphingomyelin and phos, phatidylcholine in varying mole ratio (all containing one mole dicetylphosphate per 10 moles of phospholipids) have been studied. The principal findings are: (1) Over the range 0 to 1 mole fraction sphingomyelin the liposomes exhibit multibilayer structure as visualized by electron microscopy using negative staining. (2) The two phospholipids differ in their interaction with dicetylphosphate in a bilayer structure. In mixtures of the two the effect of sphingomyelin is dominant. (3) The ability of sphingomyelin to form osmotically active liposomes depends on its fatty acids' composition. (4) Liposomes of all mole fractions of sphingomyelin are osmotically active if the C 24 : 1 fatty acid content of sphingomyelin exceeds 10% of the total acyl residues. The degree of osmotic activity, however, depends upon the molar ratio between the two phospholipids. The highest initial rate of water permeability was found for lecithin liposomes. The maximal change of volume by osmotic gradients was obtained for liposomes composed of 1 : 1 lecithin to sphingomyelin (mole ratio). ( 5) Permeability to glucose increased with increasing lecithin mole fraction. (6) Liposomes composed of 1 : 1 lecithin to sphingomyelin have the largest aqueous volume per mole of phospholipid as measured by glucose trapping. (7) The osmotic fragility of liposomes made of sphingomyelin is higher than for those made of lecithin but the highest osmotic fragility was obtained Ibr liposomes containing lecithin and sphingomyelin in 1 : 1 molar ratio. (8) When the temperature is abruptly lowered to about 2ยฐC, liposomes formed from phosphatidylcholine release about 20% of trapped glucose during a transient increase in permeability. Liposomes containing 0.5 mole fraction sphingomyelin release about 30% of the trapped glucose under these conditions. Liposomes composed of sphingomyelin alone do not exhibit this phenomenon.