By contrast immunofluorescent technique, the distribution of SC, IgA and IgA-producing cells was investigated in essentially normal gastric mucosa (19 cases) superficial gastritis (29 cases), atrophic gastritis (24 cases) and gastric carcinoma (48 cases). Excellent results were obtained, conducive to retrospective studies. Fluorescence of SC and IgA was found on the membrane of pyloric glandular epithelial cells along their sides and luminal border, but not in fundic glandular cells. IgA fluorescence alone was found in the cytoplasm of IgA-producting cells and interstices. SC and IgA fluorescence was stronger in the mucosa of superficial gastritis than in normal controls. In the glands with intestinal metaplasia SC and IgA fluorescence was markedly weakened. Among the 48 patients with gastric carcinoma, SC fluorescence was noted in 13 and IgA fluorescence in 16, demonstrated on the luminal surface in well differentiated glandular carcinoma and on the membrane or in the cytoplasm of poorly-differentiated carcinoma cells. It is concluded that some carcinoma cells still retained their function of secreting SC and ability to form SlgA irrespective of their degree of differentation.