The isoenzyme pattern of lactic dehydrogenase was measured in rectal biopsies from six patients with ulcerative colitis in whom precancerous histological changes had been independently recognized. There was a highly significant increase in the ratio of isoenzymes IV + V to I + II. This suggests that lactic dehydrogenase isoenzyme measurement may prove a valuable addition to histology in the recognition of precancer in ulcerative colitis.