Activity of transketolase and the TDP-effect were studied in blood and liver subcellular fractions of mice with Erlich ascites carcinoma and of rats with sarcoma 45 which were maintained on a synthetic diet containing either all the vitamins or devoid of thiamine. As compared with other mice liver subcellular fractions the microsomal fraction proved to be the most sensitive to thiamine deficiency: inhibition of transketolase activity reached 75%. Decrease in TDP-effect found in microsomes might reflect the most distinct terminal steps of B1 avitaminosis. As a result of vitamin B1 deprivation of mice with Erlich ascites carcinoma activity of transketolase was decreased by 30% and the TDP-effect increased by 34% in the liver microsomal fraction; in the tumoral cells the enzymatic activity was decreased by 23% and the TDP-effect was increased by 10%. Thiamine-free ration of rats with sarcoma 45, at the initial steps of the tumor growth was responsible for the most distinct decrease in transketolase activity and an increase in the TDP-effect in blood.