Superoxide dismutase (SOD) catalyzes the breakdown of superoxide into hydrogen peroxide and oxygen in the antioxidant defense system. We had reported that the SOD activities in the ceca of germ-free (GF) mice were significantly higher than those in conventional (CV) mice. In this study, we confirmed the location where SOD activity and protein expression increased in the ceca of GF mice. An immunohistochemical analysis and total SOD activity assay were conducted using the mucosa and other remaining tissues in the ceca. In addition to SOD activity in the ceca, 4 sites of intestinal (duodenal, jejunal, ileal and colonic) mucosae in GF mice were compared with those of CV mice. Total SOD activity in the cecal mucosa of GF mice was significantly higher than that in CV mice (P<0.01)and the intensity of CuZnSOD-positive cells in cecal mucosa was increased in all GF mice. Total and CuZnSOD activities in the duodenal, jejunal, ileal, cecal and colonic mucosae of GF mice were significantly higher than those in CV mice (P<0.05, or P<0.01). Furthermore, CuZnSOD mRNA showed similar tendencies with respect to these activities. Our results suggest for the first time that upregulation of SOD activity occurs in the entire intestinal mucosa of GF mice.