Recent studies have suggested that intestinal epithelial cells demonstrate some of the functions associated with immune competent cells. Based on these observations, we investigated whether gastrointestinal epithelial cells express Interleukin-6 (IL-6). The presence of this cytokine was tested in 53 normal and pathological tissue specimens of the human gastrointestinal tract using an immunohistochemical technique with anti-IL-6 monoclonal and polyclonal antibodies. Immunostaining shows that IL-6 is expressed in gastric and small intestinal epithelial cells. The tumor cells from a large subset (11 of 15) of colon cancer specimens were strongly immunostained. IL-6 immunostaining was less conspicuous and less frequent in the epithelial cells of normal colonic mucosa. Northern blot experiments indicated that the expression of IL-6 in colonic mucosa correlates quantitatively with the presence of its m-RNA. Furthermore, IL-6 receptor (IL-6R) m-RNA was also detected and was twice as abundant in colonic carcinoma as in normal colon. It is concluded that mucosal epithelial cells of the gastrointestinal system express IL-6 and that in the case of the colon, malignancy is accompanied by a higher expression. In addition, the presence of IL-6R transcript suggests that normal and neoplastic colonic epithelial cells might be autocrinally regulated by IL-6.