The tissue distribution of a previously identified M(r) 40 K epithelial autoantigen in ulcerative colitis was examined in situ in the fetal tissue by immunocytochemical method, using an immunoglobulin M (IgM) monoclonal antibody (7E12H12). Fetal-autopsy tissue from 9 to 19 weeks of development (11-21 gestational weeks) gestations, including specimens of colon (20 specimens), skin (19), small intestine (12), gallbladder (15), liver (20), spleen (20), kidney (3), urinary bladder (3), and umbilical cord/placenta (25) were examined. 7E12H12 reactive epitope appeared first in a few colonic epithelial cells at 10 weeks of gestation. By 11 weeks, 7E12H12 reactivity was clearly evident in most of the colonic epithelial cells. Goblet-cell reactivity was evident from 15 weeks of gestation. Enterocytes lining the small intestine from all the specimens were negative. In the skin, earliest immunoreactivity was evident in the basal layer of epidermis at 10 weeks of development, and by 11 weeks, the staining was intense and localized exclusively to squamous epithelial cells of the epidermis. The immunoreactivity in the fetal gallbladder mucosa was also evident earliest at 11 weeks of development. The staining was distinct in the mucosal epithelial cells, and localized along the periphery of the cells and in the apical areas. None of the other tissue specimens reacted with 7E12H12. These results demonstrate that the 7E12H12 reactive epitope is novel, is shared by colon, skin, and biliary epithelium, and appears more or less simultaneously in the three organs at 10 to 11 weeks of development of the fetus. A possible link of this crossreactive protein or proteins in the immunopathogenesis of ulcerative colitis, sclerosing cholangitis, and pyoderma gangrenosum is discussed.