Abstract.Rabbit alveolar saturated phosphatidylcholine (SPC) can be separated by differential and density gradient centrifugation of lung lavage into three fractions. One fraction (‘B’), which is analogous to conventionally prepared alveolar surfactant, contains 46·0±5·9% (SD) of lavage SPC, and is made of large multilamellar vesicles or sheet‐like structures. A second fraction (‘C’) does not sediment after centrifugation at 80000gfor 90 min, contains 29·8±14·0% of lavage SPC, and a uniform population of small vesicles. This fraction incorporates3H‐palmitate administered intravenously with a small delay with respect to fraction ‘B’. A third fraction (‘D’) contains almost all the cells of lavage and less than 5% of lavage SPC.We conclude that alveolar SPC consists of more than one compartment. The possible significance of isolat