Lamellar body material was isolated by highspeed centrifugation from the amniotic fluids of 10 mildly diseased patients followed longitudinally and of 60 other women. Three stages of development were defined: (1) immature stage: the phospholipid concentration is less than 20 µmol/l and phosphatidylcholine (PC) is approximately 50%; (2) transitional stage: the phospholipid concentration is between 18 and 40 µmol/l, of which PC is approximately 75% and phosphatidylinositol (PI) 10%. Phosphatidylglycerol (PG) is absent or present at very low concentrations; (3) mature stage: the phospholipid concentration is above 40 µmol/l; its PC percentage is 75% or more. PG is present and the percentage of PI decreases. An increase in phospholipid concentration of 3.27 ± (SD) 0.75 µmol/l/day was found during the transitional stage. The rate of increase enables us therefore to approximate the number of days that it will take before the fetal lung is ma