Using horseradish peroxidase (HRP) which serves as its own tracers, the absorption of intact macromolecules through the human gastrointestinal tracts was observed. In healthy volunteers, HRP was poured into the stomach, the duodenum, and the ileum through the gastric, duodenal and colonofiberscopes at several days interval. Twenty minutes after the injection, small samples of mucosa were removed. The tissues thus obtained were treated in the manner of Karnovsky and embedded in Epon and observed under an electron microscope. The absorption of HRP through the human small intestine and the intestinal metaplastic epithelia of the stomach was clearly demonstrated. Moreover, in intestinalized epithelium of the stomach, HRP was absorbed and transferred into the capillary lumen. These results indicated the posibility of absorption of macromolecules through the human gastrointestinal tract, which may be a cause of food-allergy and food poisoning. At the same time, it showed the effectiveness of oral administration of some enzyme-containing medicines. The absorption through the intestinal metaplastic epithelium was considered to be especially significant, because eaten proteins are promptly absorbed before digestion occurred.