Surface-active fractions similar to the biochemical constituents of the pulmonary surfactant system can be isolated from human amniotic fluid and lung tissue from week 10 of gestation through term. Amniotic fluid yielded 10 mg. per 100 ml. surface-active material at 10 to 28 weeks' gestation and 330 mg. per 100 ml. at term. Fetal lung at 10 weeks of gestation gave 1,233 mg. per 100 ml. and 1,670 mg. per 100 ml. at 4 days after birth. The constituent PC of these surface-active fractions from amniotic fluid and lung contained in excess of 60 per cent palmitoyl residues. PC associated with the surface-active fraction accounted for no more than 15 per cent of the total amniotic fluid concentration of this compound. Minimum surface tensions obtained with these preparations from amniotic fluid ranged from 2.5 dynes per centimeter at 33 weeks to 8.3 dynes per centimeter at 40 weeks. At 14 to 28 weeks, the surface-active fraction from lung gave a minimum surface tension of 18.1 dynes per centimeter and 7.0 dynes per centimeter by 4 days after birth. These findings are consistent with the observation that an increase in the activity or concentration of the fetal lung surfactant system takes place in the last 4 weeks of gestation. Some constituents of this system, or their derivatives which have possibly been altered as a result of biochemical changes, are found in amniotic fluid. We infer that the majority of PC in amniotic fluid is not directly associated with the biochemical constituents of the fetal lung surfactant system.