Measurement of lactic dehydrogenase activity of basal gastric juice was attempted in order to evaluate the validity of the method as a tool for the diagnosis of various gastric disorders. The activity, however, was found to show a remarkable decrease in gastric juice with a PH below 6.0 forthis reason, intragastric neutralization with 3.5% sodium bicarbonate solution was performed in the present work. Patients consist of 12 cases of gastric cancer, 11 of chronic gastritis, 8 of benign peptic ulcer, and 8 of normal controls. Specific activity of LDH in normal controls ranged 1 to 10 units per milligram of soluble protein, with a mean value of 4. It ranges 1 to 13 with a mean value of 4 in benign peptic ulcer, 5 to 25 with that of 14 in chronic gastritis, and 8 to 74 with that of 25 in gastric cancer. The results indicate that the specific activity of LDH in gastric juice is specifically elevated in gastric cancer and somewhat less in chronic gastritis, and that the method may be used as one of supplementary diagnostic tools of gastric cancer. Furthermore, it is suggested that the elevated specific activity of LDH in gastric cancer and chronic gastritis might have derived from the identical origin.