An animal model of Helicobacter pylori infection was obtained with Mongolian gerbils. Culture broth of H. pylori ATCC 43504 was orally administered to the gerbils. Animals were killed 1, 7, 14, 21, and 42 days after the oral inoculation and the stomachs were excised. Colonization of H. pylori was maintained in the stomachs of all animals throughout the study period of up to 42 days. On microscopic examination after immunological and hematoxylin and eosin staining, the organisms were detected in the mucus layer of the gastric epithelium. All gerbils exhibited a slowly progressive but typical gastritis, showing superficial erosion with increasing numbers of infiltrating neutrophils and mononuclear cells. Forty-two days after the inoculation, lymph follicles were detected in the gastric submucosa. These features are similar to the manifestations of H. pylori infection in humans and, accordingly, this model could be of value for studying H. pylori pathogenicity and for research into agents for the treatment of H. pylori infection in humans.