Studies were carried out by contrast immuno-fluorescent staining on the distribution of D and G cells in the gastric antrum of patients with gastric and duodenal ulcer, chronic gastritis or normal antral mucosa. No significant differences were found iii the number of D or G cells of duodenal ulcer patients and normal controls, but in those with comparable degrees of antral atrophic gastritis, G cells seemed to be more numerous in patients with duodenal ulcer than in those without. The percentage of G cells containing more gastrin (as judged by the intensity of fluorescence) was much higher in the duodenal ulcer group. This may indicate that G cell with augmented activity played an important role in stimulating secretion of gastric acid in these patients. The number of D and G cells in gastric ulcer patients was significantly reduced, but the ratio between D and G cells was essentially the same in both gastric ulcer patients (3.37) and duodenal ulcer patients (3-17). This demonstrates that the quantitative variation of D cells may not be an important factor in maintaining gastric acidity.