AbstractTurnover of [14C]thiamin was studied in mice with Ehrlich ascites carcinoma fed a thiamin‐deficient diet and injected with 5 or 2μg/mouse of the labeled vitamin. The process of conversion of [14C]thiamin to thiamin pyrophosphate (TPP) was monitored by measuring the activities of transketolase, pyruvate dehydrogenase, and oxoglutarate dehydrogenase. The amount of coenzyme‐unsaturated apotransketolase was assessed by measuring the TPP effect—determining transketolase activity with and without the addition of TPP in vitro.Tumor growth was accompanied by thiamin deficiency, manifested in an increase in [14C]thiamin incorporation into the host tissues and the absence of saturation of the tissues with the labeled vitamin over 13 days. Increased values for the turnover coefficients, reduction of thiamin‐dependent enzyme activities, elevation of the TPP effect, and a decrease in urinary excretion of the radioactive products also provided evidence for a disturbance in thiamin metabolism. The severity of the disturbance in thiamin metabolism during malignant tumor growth was directly related to the dose of the exogenous vitamin.(Nutr Cancer 6, 260–273, 1985)