The distribution of human intestinal 15 kDa protein (I-15P), a new fatty acid-binding protein (FABP), was observed in normal tissues using immunohistochemical techniques. The antiserum against human I-15P intensely reacted with the villous epithelium of the terminal ileum but not with the enterocytes of the crypts. Although the surface epithelium of the stomach and villous epithelium of the duodenum showed weak reactivities, the epithelial cells of the jejunum, proximal ileum, colon and rectum, and also glandular epithelia with intestinal metaplasia of the stomach were not immunostained. The other human tissues examined were negative for anti-human I-15P antibody. Human I-15P thus represents a distinctive, confined tissue distribution different from the other FABPs and is expected to serve as a useful cellular marker of terminal ileal enterocytes.