In order to determine whether the LDH isoenzyme pattern, measuring the percentages of LDH4-5 (M monomer), might be a marker in the adenoma-carcinoma sequence, 103 adenomas, 8 adenomas with proven malignant degeneration and 27 adenocarcinomas of the large intestine, as well as 12 biopsy samples of colonic mucosa from normal controls, were studied histologically and histochemically. The proportion of M polypeptide was significantly increased in adenomas of larger size (diameter > 2 cm), in adenomas with a larger villous component, and in those with severe dysplasia. The largest proportion of M polypeptide was found in villous adenomas and colloid adenocarcinomas, and might reflect a common origin. These results suggest that the histochemical study of the LDH isoenzymogram might be a useful marker for detecting early malignant degeneration in adenomas of the colon.