Data linking interactions between bacteria and the intestine with elevated serum cytokine levels has led to the concept of the gut as a cytokine-producing organ. An in vitro cell culture model was used to investigate the potential role of intestinal mucosa within this paradigm. Polarized monolayers of human enterocytes (Caco-2) were grown in a two compartment system where the apical and basal aspects of the membrane could be studied. Supernatant was collected at 0, 1, 3, 6, and 24 h after the monolayer was exposed (apically or basally) to 10(2), 10(5), or 10(8) colony-forming units of Escherichia coli C25/mL and saved for interleukin (IL)-6 and tumor necrosis factor (TNF) bioassay analysis. Caco-2 cells (not bacterially challenged) secreted significant amounts of constitutive IL-6, but not TNF, into the apical and basal chambers. Both cytokines levels were increased in a dose-dependent fashion (p < .05) after the E. coli challenge. This stimulated cytokine response was polar, in that the highest cytokine levels were at the side of the bacterial challenge and were most notable at the highest dose (10(8) colony-forming units/mL) of E. coli C25 tested. Caco-2 cells produce IL-6 and TNF in a dose-dependent fashion in response to E. coli C25 and the magnitude of this response is maximal on the side of the bacterial challenge. This data supports the hypothesis that bacterially challenged human enterocytes may be important producers of cytokines.