An assay has been developed and evaluated that quantifies the surface tension lowering ability of amniotic fluid surfactant. The formation of stable foam following vigorous shaking of amniotic fluid was evaluated by the addition of various amounts of dipalmitoyl lecithin in a solution of ethanol and saline and by fine adjustments of the ethanol volume fraction in the final assay mixture. The foam stability index (FSI) for a particular sample of amniotic fluid was defined as the highest ethanol volume fraction that would permit the formation of stable foam after vigorously shaking a mixture of ethanol and amniotic fluid. The assay is referred to as the FSI test. We report the FSI values in amniotic fluid specimens from 59 patients obtained within 72 hours of delivery. The L/S ratio was measured in SO of the same 59 specimens. We observed 6 cases of neonatal hyaline membrane disease (HMD) and 2 cases of transient tachypnea of the newborn (TTNB) in this study. No cases of HMD or TTNB occurred with FSI values of greater than 0.47, while 2 cases of HMD were recorded in association with L/S ratios of 2.5 and 2.8, respectively. The potential clinical value of the FSI test is discussed.