The chemical nature of phospholipids in rat lung tissue and alveolar wash was studied using column chromatographies on silicic acid and DEAE-cellulose and thin-layer chromato-graphy followed by gas chromatography to determine the fatty acid composition of the major phospholipid fractions. It was found that alveolar phospholipids obtained by lung lavage differed from those in lung tissue in several aspects. Predominant phospholipid from both sources was lecithin which composed 68% of alveolar phospholipids and 52% of lung tissue phospholipids. Alveolar lecithin contained much higher disaturated species than lung tissue. Lipids X₁, X₂ and X₃ were found in higher proportions in alveolar wash. The X₃ seemed to be identical with
cardiolipin from the data of Rf values of intact lipids as well as deacylated products, and elution pattern with DEAE-cellulose. The acidic lipid X₂ seemed to be probably phosphatidylglycerol. Lipid Xl could not be identified. The present results suggested that lecithin and the three acidic phospholipids could be secreted more preferentially into alveolar cavity than other lipids, such as neutral lipids, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol and sphingomyelin.