By using an original method of electrophoresis separation, the normal relative values of each one of the LDH isoenzymes in the gastric body and antral mucosa were determined. It has been shown in gastric tumors that there is a significance rise in the relative values of "type M" isoenzymes (iso-LDH-4-5). With intestinal metaplasia as well as gastritis, a significant elevation of the relative value of iso-LDH-5 has been demonstrated in the antral mucosa. In 6 benign gastric polyps we found a similar disturbance. This would confirm the known precancerous condition of these lesions. In the normal mucosa of subtotal gastrectomy specimens, 19 patients with antral cancer had 2 types of LDH isoenzyme patterns: one normal and the other with an increase in the relative proportion of "type M" isoenzymes. These LDH isoenzyme pattern shifts in the gastric mucosa could be an early sign of malignancy prior to the morphologic changes.