a1-Antitrypsin and a1-Antichymotrypsin were demonstrated by the peroxidase--antiperoxidase (PAP) technique in paraffin sections from various parts of the normal adult gastrointestinal tract. Thus, positive reaction for these two proteinase inhibitors was given by the absorptive epithelial cells of the small intestine and epithelial cells of Brünner's duodenal glands and the glands of the pyloric antrum. Our findings suggest that the presence of a1-Antitrypsin and a1-Antichymotrypsin in epithelial cells of the small intestine, may represent a self-protecting mechanism against the pancreatic proteolytic enzymes of the intestinal juices. The significance of our observation and its possible role in the previously reported a1AT deficiency associated with coeliac-like disease is also discussed.