Pulmonary surfactant is the material which, by lining the lung alveoli, prevents their collapse during expiration. The ability of this material to lower surface tensions, which would otherwise be generated by the presence of a fluidgas interface, in the alveolus has led to the designation 'surfactant'. Without surfactant alveoli cannot remain expanded and undergo collapse. Perhaps the most dramatic demonstration of the importance of lung surfactant concerns respiratory distress syndrome in newborn infants where the high surface tension of lung extracts is caused by a deficiency of surfactant. This deficiency is, in turn, caused by the immaturity of the alveolar epithelial type I1