Cell proliferation of human gastric mucosa was studied in the cases of chronic gastritis using the in vitro incubation method of 3H-thymidine autoradiography. The study was carried out using the material consisted of 92 biopsy specimens and 83 stomachs diagnosed as carcinoma, peptic ulcer, duodenal ulcer and chronic gastritis.Fresh gastric mucosae were taken from the antrum and pylorus of the stomach under gastroscopic observation and also from extirpated stomachs. The tissues were trimmed into rice-grain-sized pieces and were incubated for one hour at 37.. in Eagle medium and 2μC of 3H-TdR.The gastric mucosae were divided light microscopically into five subtypes by Hirafuku criteria as the following: superficial, simple, atrophic hyperplastic, atrophic and metaplastic gastritis.The labeled cells were ordinally observed in the proliferative compartment, which was situated in the glandular neck region and active mitotic figures.The labelling index was expressed in a percentage of labeled cells in ratio to the total number of epithelial cells. In the normal gastric mucosae, 3H-TdR labeled cells were in the neck region of the gastric gland, but did not appear in the surface epithelium. Higher in corporation of 3H.TdR was observed in the lower part of the neck region of the glands. The average indices, both labeling and mitotic, were generally higher in the antrum than in the pylorus in the cases of chronic gastritis and also higher than normal mucosae.Superficial gastritis showed many labeled cells which were located in the neck region and foveolae. Simple gastritis showed scattered labeled cells in various parts of mucosae. In atrophic and atrophic hyperplastic gastritis, labeled cells were found in the neck and foveolae of the gastric glands. Metaplastic gastritis showed labeled cells especially in the neck regions. The average labeling index is higher in simple chronic gastritis than in other superficial gastritis, atrophic, atrophic hyperplastic and metaplastic gastritis. Information concerned with cell renewal and proliferation is important for further understanding of the development of disease.