The rate of gastric epithelial cell proliferation was studied in healthy volunteers and in patients with different degrees of gastritis. Endoscopic biopsies from the antral and fundic part of the stomach were incubated in vitro with 3H-thymidine for 30, 120, and 210 minutes respectively. Autoradiographs were prepared, and the percentage of DNA-synthesizing cells (labeling index) in the progenitor cell region was estimated. From the successive labeling indices the rate of entry of cells into DNA-synthetic phase (S-phase) and the duration of the S-phase could be estimated. All the biopsies were classified according to the degree of gastritis. The mean (+/-SEM) length of the S-phase was found to be 7.4 +/- 0.3 hours in antral mucosa and 7.2 +/- 0.4 hours in fundic mucosa. There was no significant difference between the S-phase duration in normal mucosa, superficial gastritis, mild atrophic gastritis and severe atrophic gastritis. This observation suggests that the labeling index can be used as an expression for the rate of cell proliferation in human gastric mucosa. A significant correlation between the labelling indices and the degree of gastritis was found in both antral and fundic mucosa. In six cases, labelling indices estimated by cell counts performed on longitudinal or cross sections of foveolae were compared. Ther was no significant difference between the results obtained by the two different counting techniques.