Media from bacterial cultures have been tested for actions on prostaglandin biosynthesis by human amnion and decidual cells. The bacterial species, which are commonly associated with intrauterine infections, were Group B streptococcus, Escherichia coli, Fusobacterium nucleatum, Bacteroides fragilis, Gardnerella vaginalis, Neisseria gonorrhea, Mycoplasma hominis and Ureaplasma urealyticum. Overall, low doses of bacterial products were stimulatory of amnion prostaglandin production, whereas high doses were inhibitory. A similar pattern of results was obtained for effects on decidual prostaglandin production, although stimulatory actions at low doses were less pronounced. In all experiments interleukin 1 beta consistently induced a stimulation of prostaglandin production that greatly exceeded that caused by any bacterial product. It is possible that the inhibitory action of high doses of bacterial products on prostaglandin biosynthesis may contribute to the poor course of labor experienced by women with chorioamnionitis. Furthermore, these data lend credence to the view that the host response to infection (i.e. cytokine secretion) is the major mediator of subsequent preterm labor.