ExtractA simple technique, precipitation with acetone, was described to separate the surface-active lecithin fraction from the nonsurface-active fraction. Surface activity in lung phospholipids was found in the acetone-precipitated fractions of lecithin, sphingomyelin, phosphatidyl dimethylethanolamine and phosphatidyl inositol. Normal surface activity of saline extract of pooled fetal rabbit lung was observed from 28 days of gestation. It was possible to isolate surface-active lecithin from lung parenchyma long before the 29th day of gestation when surface-active lecithin first is found in the alveolar wash. During the nonbreathing fetal state, even at term, only 11% of lecithin from alveolar wash is surface-active increasing after one hour's breathing to approximately 50% of the total lecithin. The rabbits delivered prematurely after 28 full days of gestation clinically had respiratory distress and their percentage of surface-active lecithin in alveolar wash increased at a slow rate compared to full-term animals. Good temporal correlation was seen between intracellular storage of surface-active lecithin during the fetal state and the findings with electron microscopy of increasing numbers of osmiophilic inclusion bodies as gestation progresses.SpeculationSurface activity is shared by several phospholipids in lung but is related principally to lecithin. During fetal development there is production of intracellular surface-active lecithin with storage possibly in osmiophilic lamellar inclusion bodies until near term when some (11%) begins to appear in alveolar wash. After breathing, a great release of surface-active lecithin into alveolar wash occurs, with 50% of alveolar lecithin being surface active throughout the life of the animal. Prematurely delivered rabbits take much longer to increase their surface-active alveolar lecithin.