The presence of fetal lung surfactant in amniotic fluid is indicative that fetal lungs are sufficiently mature for delivery without subsequent respiratory distress. However, current methods for detecting surfactant fail to positively distinguish saturated phosphatidylcholine (primarily dipalmitoyl phosphatidlycholine) from the many similar phosphatidylcholines also present in amniotic fluid. As a result small amounts of this lipid are obscured by the presence of the other lipids and delivery of many patients is delayed unnecessarily. A sensitive method is here described in which extraneous phosphatidylcholines are oxidized while the saturated phosphatidylcholines remain intact for positive determination by thin-layer chromatography. The method is insensitive to contaminating lipids from blood or meconium and provides an unequivocal test for the presence of disaturated phosphatidylcholine in a relatively short time. About one half of the patients tested in whom lecithin/sphingomyelin ratios were 1.4 to 1.7 had this surfactant lipid present in their fluids and presumably could have been delivered immediately.