Pulmonary surfactant was isolated from human amnionic fluid by use of differential speed centrifugation and discontinuous density gradient centrifugation. Its chemical composition and physical properties were determined. Phospholipid was found to be the main component, and the chief substance causing reduction of surface tension. The minimum surface tension could be reduced to 7.2 mN/m. The compressibility was 0.039 mN/m, when surface tension was 10 mN/m. The surface concentration of phospholipid was 1.086 ...