The present study evaluated the secretions of interleukin (IL)-1β and tumor necrosis factor (TNF) α by fetal membranes stimulated with group B streptococci (GBS) and lipopolysaccharide (LPS). The aim was to evaluate the initial response of full-thickness membranes to the microbial insult using an in vitro experimental model that allowed testing of the individual contributions of amnion and choriodecidua to stimulation. Full-thickness membranes were obtained after delivery by elective cesarean section from women at 37–40 wk of gestation without evidence of active labor. The membranes were mounted in Transwell devices, physically separating the upper and lower chambers. The LPS (500 ng/ml) or GBS (1 × 10⁶ colony-forming units/ml) was added to either the amniotic or choriodecidual surface, and accumulation of IL-1β and TNFα were measured in both compartments using a specific ELISA. Fetal membranes followed different patterns of secretion of proinflammatory cytokines that depended on the side to which the stimulus was added or the nature of the stimulus itself. The TNFα was secreted by amnion and choriodecidua in the presence of LPS or GBS, and stimulation with GBS induced a greater synthesis of IL-1β than did stimulation with LPS. Choriodecidual tissue was more responsive than amniotic tissue, and this response tended to be higher even when the stimulation was only on the amniotic side. However, the amnion plays an active role in recognizing LPS or GBS, contributing a significant amount of TNFα. Thus, cooperative and bidirectional communications occur between amnion and choriodecidua in response to bacterial products, which include intermembranous cytokine traffic and signaling between tissues.