The mechanisms responsible for the onset and progression of preterm labor are poorly understood. In the present study a total of 115 amniotic fluid specimens were analyzed for a lipid that has not previously been detected in amniotic fluid. This glycolipid was identified as ceramide lactoside. It was found in two-dimensional thin-layer chromatograms for evaluation of lung maturity and quantified by gas chromatography. Ceramide lactoside concentrations in amniotic fluid were low in spontaneous labor at term (1.7 +/- 0.7 nmol/ml) and in pregnancies that were not associated with spontaneous preterm labor (1.4 +/- 0.6 nmol/ml). The concentrations were high in chorioamnionitis with signs of infection (11.8 +/- 5.8 nmol/ml) and in preterm labor without clinical signs of chorioamnionitis (5.4 +/- 4.0 nmol/ml). A high ceramide lactoside (greater than or equal to 5 nmol/ml) predicted chorioamnionitis with signs of infection at a sensitivity and a specificity of 94% and 95%, respectively. A moderately high ceramide lactoside concentration (greater than or equal to 2.5 nmol/ml) predicted spontaneous preterm labor: sensitivity, 82%; specificity, 95%. Little, if any, ceramide lactoside was present in urine, vernix, normal fetal membranes, or lung effluent, whereas this glycolipid was present in large amounts in granulocytes and in inflamed fetal membranes. We propose that phagocytosing granulocytes release ceramide lactoside into amniotic fluid.