The distribution and cellular location of substances reacting with anti-glucagon or anti-glicentin sera, i.e., glucagon-like and glicentin-like immunoreactivities, were studied in the human digestive tract using the immunofluorescence and immunoperoxidase methods. Both types of immunoreactivity were (1) absent in the antrum, (2) abundant in cells located at the periphery of pancreatic islets, (3) unevenly present in cells scattered in the epithelium of the small intestinal mucosa, the glicentin-immunoreactive cells being particularly abundant in the ileum. In the pancreas, and, when simultaneously present, in the intestine, both glucagon and glicentin immunoreactivities were located in the same cells. The precise ultrastructural location of each immunoreactivity was readily made using colloidal gold and ferritin tracers on ultrathin sections of glutaraldehyde-osmium fixed and epoxy resin-embedded tissues. In the pancreas, both glucagon and glicentin immunoreactivities were found in the granules of the A-type cells; the glucagon immunoreactivity was only present in the core of the granule, whereas the glicentin immunoreactivity was found either in the peripheral halo only, or throughout the entire granule. In the small intestine, both immunoreactivities were located inside the granules of the L-type cells. Quantitative specificity tests suggested that the glucagon- and the glicentin-like substances of the pancreas differ from those found in the intestine.