Total pepsin activity and gastric acid secretion have been estimated in the resting basal and maximally stimulated gastric juice in groups of normal controls, patients with gastric ulcer, duodenal ulcer and gastric carcinoma. Stimulated pepsin and acid secretion showed a significant stepwise reduction from patients with duodenal ulcer to normal controls, patients with gastric ulcer and those with gastric carcinoma. Electrophoresis of pepsin in all samples of gastric juice was undertaken and showed differences in the distribution of pepsin bands in patients with gastric carcinoma compared with all other groups. Pepsin band 6 was seen almost exclusively in these patients and did not occur in any normal controls. Changes in pepsin banding were more marked when extensive atrophic gastritis was present in the stomach. This study suggests further investigation of pepsin should be undertaken to determine the relationship of these changes to gastric malignancy.