Immunohistochemical analysis of normal human intestine revealed that two anti-CD23 monoclonal antibodies (mAb), EBVCS 1 and EBVCS 2, reacted with human intestinal epithelial cells. Both mAb exhibited an exclusive reactivity with epithelial cells of the small and large bowels. Staining with both EBVCS 1 and EBVCS 2 was localized on the apical and basal sides of enterocytes. Enhanced expression of CD23 on gut epithelial cells was found in inflammatory bowel diseases, in children with food intolerance to cows' milk proteins and in a young infant with severe autoimmune enteropathy. Western blot analysis of anti-CD23 mAb reactivity with gut epithelial cell extracts showed the presence of a non-reducible 42,000-45,000 M (r) polypeptide compatible with the membrane form of the intact CD23 molecule. These data show that CD23 is constitutively expressed by intestinal epithelial cells and that its expression is enhanced in enteropathies.