Sections of ethanol-fixed, paraffin-embedded tissue specimens from different parts of the human gastrointestinal tract were stained by an indirect immunofluorescence method with a rabbit antiserum to HLA-DR-antigens from B lymphocytes. A specific staining reaction was seen in a patchy pattern apically in the columnar cells of the normal small intestine, decreasing in intensity from the top of the villi towards the crypts. No HLA-DR-like antigens could be detected in colon or stomach epithelium, whereas cells with the morphology of lymphocytes and histiocytes in the lamina propria and also capillary walls were specifically stained throughout the gastrointestinal tract.