To further investigate the interaction between human mononuclear leucocytes [peripheral blood mononuclear cells (PBMC)] and enterocytes, the effect of a confluent layer of differentiated CACO-2 cells on cytokine kinetics during challenge with bacteria in a compartmentalized coculture model was investigated. Nonpathogenic Escberichia coli were added either to the apical or the basolateral compartment of this transwell cell culture system, the latter of which contained human leucocytes. The synthesis of tumour necrosis factor (TNF-α) and interleukin (IL)-12 was significantly suppressed by CACO-2 cells when leucocytes were stimulated directly with bacteria. This suppression was not paralleled by changes in the production of IL-10, IL-6 and transforming growth factor (TGF)-β. When the bacteria were applied apically to the CACO-2 cell layer, the production of TNF-α, IL-12, IL-1β, IL-8, IL-6, IL-10, TGF-β and interferon-γ was pronouncedly lower as compared to the bacterial stimulation of leucocytes beneath the CACO-2 cells. In the latter experiments, IL-6, IL-8 and TNF-α were the cytokines being mostly induced by apical addition of E. coli. Quantitative mRNA expression analysis revealed that IL-8 gene expression was equally induced in both CACO-2 and PBMC after apical stimulation with bacteria. Of note, bacteria-stimulated CACO-2 cells produced little or no cytokines in the absence of leucocytes, supporting the concept of leucocyte-epithelial cell cross-talk in modulating cytokine responses in the gut mucosa.