A total of 206 determinations of amniotic fluid total phospholipids (TPL) and L/S ratio have been made at varying stages of gestation in normal pregnancies, preterm deliveries, and pregnancies complicated by diabetes mellitus, hypertensive disorders, and placental insufficiency. TPL concentration and L/S ratio demonstrate similar increases during gestation to term. From the thirty-sixth week onward, 1.1 per cent of L/S ratios were less than 2 and 3.4 per cent of TPL determinations were less than 2.5 mg. per 100 ml. In pregnancies complicated by diabetes mellitus, hypertension, and placental insufficiency, a TPL concentration of 2.5 mg. per 100 ml. or more is well correlated with an L/S ratio of 2 or more and the fetal outcome. In preterm deliveries, all cases with L/S ratio of 2 or more did not develop RDS. This value of L/S ratio was well correlated with a TPL concentration of 2.8 mg. per 100 ml. or more. L/S ratio below 1.5 and TPL below 1.6 mg. per 100 ml. were associated in all cases with RDS. The intermediate zone between these values, where fetal respiratory outcome could not be predicted, contained significantly more cases when TPL was evaluated compared to evaluation by the L/S ratio. It would appear that TPL determination is valuable if values are more than 2.8 or less than 1.6 mg. per 100 ml. Between these values L/S ratio determination is advised.