The content of14C-thiamine and activity of the thiamine-dependent enzyme transketolase (TK), and the thiamine diphosphate (TDP) content were studied in mouse tissues during the development of Erhlich's ascites tumor. The TDP concentration in the liver of animals with tumors fell steadily for 10 days after inoculation, and the TDP level in the tumor itself fell more steeply toward the terminal period of tumor growth (10th day). Meanwhile a sharp deficiency of coenzyme was found in the tumor, but there was an excess of apoenzyme forms of TK. The increasing thiamine deficiency in the tumor placed a greater strain on thiamine metabolism and led to the appearance of competitive relations between tumor and host, which were manifested most clearly against a background of hypovitaminosis.