1. Phenotypical heterogeneity of macrophages infiltrating the colonic mucosa of patients with ulcerative colitis has been shown in many reports. The functional diversity of macrophages in inflamed colonic mucosa remains unclear. 2. Intestinal macrophages have been characterized by immunohistochemical staining and their ability to generate free oxygen radicals in inflamed and non-inflamed mucosa. 3. No correlation was found between RFD1 (activated dendritic cells), RFD7 (tissue macrophages) or RFD9 (epithelioid cells, giant cells in association with granuloma) positivity and either CD68 or human histocompatibility complex class II antigen (HLA-DR) positivity. The proportions of RFD7- and RFD9-positive cells were significantly increased in inflamed colonic mucosa. Nitroblue tetrazolium-reducing mononuclear cells were CD68 or RFD7 positive but they were rarely positive for HLA-DR and RFD1. 4. These results suggest that nitroblue tetrazolium-reducing macrophages are (scavenger) macrophages.