The production of reactive oxygen species may have an important role in the pathogenesis of inflammatory bowel disease, yet the cells responsible in colonic mucosa have not been clearly defined. We studied 28 patients with ulcerative colitis and 13 controls to determine the cells generating reactive oxygen species, using a combined method for determining nitroblue tetrazolium reducing activity and immunohistochemical characterization. In contrast to the proportion of EBM11-positive cells (tissue macrophages), which did not increase in inflamed mucosa, the proportions of PMN-13F6 positive cells, CD11b-positive cells, and eosinophils were significantly increased in inflamed mucosa. PMN-13F6 positive cell and eosinophil counts were significantly correlated with CD11b positivity. Eosinophils, however, showed a stronger correlation with CD11b positivity than PMN-13F6-positive cells. The majority of CD11b-positive cells were eosinophils. Although some nitroblue tetrazolium reducing leukocytes were positively stained with EBM11 or PMN-13F6, eosinophils were the major subset of nitroblue tetrazolium reducing leukocytes. Recruitment and activation of eosinophils may play an important role in the pathogenesis of ulcerative colitis.