THOUGH Neergaard pointed out thirty-five years ago1 that the properties of pulmonary alveolar surfaces must influence the function of the lungs and Macklin2 suggested more than ten years ago that certain alveolar epithelial cells may regulate these properties, their proposals received little attention. More recently it was deduced3 from the behavior of microscopic bubbles removed from the lungs that the alveoli are lined with a substance that greatly reduces surface tension, and the presence of such material (surfactant) in saline extracts of normal lungs was directly demonstrated.4 Subsequently, it was shown that extracts prepared from the lungs of infants dying . . .