Studies on the lecithin: sphingomyelin (L:S) ratio in amniotic fluid were made before and after the addition of meconium in 130 normal pregnancies. The meconium was collected from premature and term infants after delivery. That which was collected from premature neonates had no significant effect on the mean L:S ratio of amniotic fluid; in 28% (17 of 60), however, the L:S ratio was elevated by 0.1 to 0.2, though in none by more than 0.2. When meconium collected from term neonates was added to amniotic fluid, it significantly increased the mean L:S ratio of amniotic fluid when the baseline L:S ratio was between 1.0 and 1.9 (P less than .01); in 56% (39 of 70), the L:S ratios were elevated by 0.1 to 0.5, though none increased by more than 0.5. This study suggests that when amniotic fluid is contaminated with meconium, an L:S ratio of 2.2 in premature pregnancies and 2.5 or greater in term pregnancies retains its original predictive value.