There is a strong association between preterm labor and infection. Some potentially pathogenic bacteria have phospholipase activity, and it has been suggested that release of phospholipase from these organisms may increase prostaglandin E2 synthesis in amnion cells and hence initiate preterm labor. In this study we established monolayer amnion cell cultures from tissue collected at elective cesarean section at term before labor. Cells were prelabeled with tritiated arachidonic acid and then further incubated after addition of 2%, 5%, or 10% (vol/vol) filtered medium in which either group B beta-hemolytic streptococcus, Streptococcus viridans, Escherichia coli, Bacteroides fragilis, or Lactobacillus had been growing. Tritiated arachidonic acid and its metabolites released by the amnion cells in these or control incubates were extracted from culture medium and separated by high-performance liquid chromatography. Addition of conditioned medium from each of the organisms with the exception of Lactobacillus caused an increase in overall arachidonic acid metabolism. There was an increase in the ratio of cyclooxygenase to lipoxgenase metabolism and in prostaglandin E2 production in particular when compared to controls. The profile of arachidonic acid metabolism in amnion cells following addition of filtered bacterial medium resembled that obtained from amnion cells cultured following spontaneous labor. We suggest that abnormal bacterial colonization of the genital tract may lead to an increase in arachidonic acid metabolism in amnion cells with an increase in prostaglandin E2 production and the consequent initiation of preterm labor.