Blood of patients with gastric tumor was studied after their admission to the hospital and after the chemotherapeutic course. Formation of the tumor was accompanied by development of hypovitaminoses B1 and PP. The vitamin deficiency was more distinct after treatment of the patients with cyclophosphan: content of thiamine diphosphate (TDP) was decreased by 40%; NAD+NADP, by 30% and NADH+NADPH, by 20%. In mice with Ehrlich ascites carcinoma, activity of transketolase in erythrocytes was decreased by 48%, content of TDP, by 61% and that of NADPH, by 27%. The administration of cyclophosphan increased further thiamine deficiency in the tumor-bearing mice. Simultaneous administration of thiamine and cyclophosphan abolished the cytostatic toxic effect but did not affect their antitumoral properties. Under these conditions treatment with vitamins B1 and PP complex was undesirable due to malignization. The vitamins B1 and PP did not stimulate the tumor growth, partially restored impaired metabolism of the vitamins and may be included separately into combined multidrug oncotherapeutics.