The distribution of immunoglobulin (Ig)- containing cells in the gastric antral mucosa of patients with various abnormalities of the stomach was studied by fluorescent antibody techniques. IgA-containing cells comprised about 80% of all Ig-containing cells and were diffusely distributed in the interstitial spaces of gastric glands. IgA was observed in the apical portion of the cytoplasm of cells in some pyloric glands, and IgM was seen extracellularly between the epithelial cells and in the lumen. At the base of gastric cancers, adjacent to the invading cancer mass, IgG-containing immunocytes were found in intermuscular spaces, and some cells that appeared to be cancer cells reacted with anti-IgG or anti-IgM.