The LDH isoenzyme pattern of the uninvolved mucosa of gastric cancer patients differs as regards the LDH isoenzyme pattern from that of similar tissue of patients with benign gastric disease; the former tissue is characterised by a high M/H ratio of the LDH isoenzymes (M and H sub-units). A high M/H ratio characterises antral mucosa when the latter is compared with fundic mucosa. Mucosa showing superficial and atrophic gastritis also has a higher M/H ratio, whereas the presence of intestinal metaplasia does not appear to influence the M/H ratio. These observations are consistent with the concept that the tissue from which the cancer arose may possess a pre-malignant biochemical lesion.