Rats infected with the hepatic metacestode Taenia taeniaeformis develop an extraordinary gastric hyperplasia. Indirect immunoperoxidase staining localized larval in vitro excretory secretory product ...specifically in the supranuclear cytoplasm of the epithelial cells lining the pits and glands in the hyperplastic gastric mucosa. The accumulation of this substance in the stomach epithelial cells may be relevant to the gastric hyperplasia induced by tapeworm infection.