Oral administration of cholera toxin (CT) induces a strong mucosal immune response to CT as well as having a potent adjuvant effect. Since one of the first cell types to encounter CT during cholera infection or after oral administration is the epithelial cell, we studied the effect of CT on interleukin-6 (IL-6) secretion by the rat intestinal epithelial cell line IEC-6. CT was found to rapidly enhance IL-6 secretion and IL-6 gene expression by these cells. The addition of dibutyryl cyclic AMP (cAMP) to cultures of IEC-6 cells had little effect on IL-6 secretion, yet mRNA levels were elevated, suggesting that the response may have been regulated by cAMP. Purified B subunit of CT did not significantly enhance IL-6 secretion or mRNA expression. CT and transforming growth factor β1 synergistically enhanced IL-6 secretion in IEC-6 cells. The addition of CT with either IL-1β or tumor necrosis factor alpha gave even greater synergistic enhancement of IL-6 secretion, and dibutyryl cAMP could mimic CT's synergy with IL-1β. These results indicate that the intestinal epithelial cell is capable of secreting high levels of IL-6 after encountering CT, especially in the presence of inflammatory cytokines. This high level of IL-6 secretion could be a very important component of the mucosal immune response to CT and may also account for a portion of the adjuvant effect of CT.