A characteristic feature of histologic chorioamnionitis caused by ascending infection is the amniotropism displayed by polymorphonuclear leukocytes in the placental membranes. We tested the hypothesis that amniotic fluid from patients with histologic chorioamnionitis and intact membranes without clinical infection contains leukoattractants detectable by an in vitro leukotactic assay. Leukoattractants were detected in 13 cases (87%) and absent in 2 cases (13%) with histologic chorioamnionitis (p less than 0.005). The two cases with histologic chorioamnionitis that had negative leukotaxis were classified as stage l/mild severity. A positive leukotactic response was a better predictor of histologic chorioamnionitis (87%) than positive Gram stain (33%), amniotic fluid culture (53%), gas-liquid chromatography (40%), or a combination of these three methods (60%). These results suggest that demonstration of leukoattractants in amniotic fluid is an earlier and more sensitive predictor of chorioamnionitis than is presently available.