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The ultrasonic absorption of liposomes was determined as a function of temperature using a cylindrical resonator. The liposomes were composed of a 4to1 mixture by weight of depalmitoyl phosphatidylcholine and dipalmitoyl phosphatidylglycerol. A large increase in absorption occurs at the gel to liquid-crystalline phase transition temperature. The diameters ranged from 20 to 200 nm for single bilayer vesicles and from 100 to 800 nm for multilayer vesicles. This range eliminates the gap in data which existed between the very small and multilayered vesicles. The absorption peak decreases in magnitude with decreasing liposome size and, at the smallest sizes, increases in width and occurs at a lower temperature. The peak decreases in magnitude and increases in width with the addition of cholesterol. (