It has previously been demonstrated that decarboxylation of ornithine in tumors, and the oxidative splitting of N1-acetylspermidine in tumor and normal tissues, are important sources of putrescine. Both these sources are utilised by tumors and other tissues with a high demand for polyamines to ensure their polyamine requirement. Consequently, combined treatment of tumor-bearing animals with an inhibitor of ornithine decarboxylase (e.g. alpha-difluoromethylornithine) and polyamine oxidase (e.g. N,N'- bis-allenylputrescine) has an antitumoral effect superior to that of either drug alone. In the present work, it was demonstrated that the alimentary tract is a third important source of polyamines which maintains tumor growth. Gastrointestinal polyamines are of alimentary origin, and are also formed by aerobic and anaerobic microorganisms. They can be reduced by feeding a polyamine deficient diet together with antibiotics that are suitable for decontaminating the gastrointestinal tract. This treatment combined with the administration of the mentioned inhibitors of ornithine decarboxylase and polyamine oxidase completely prevents Lewis lung carcinoma from growing, and prolongs considerably the average life span of L1210 leukemia mice. The results of the polyamine analyses of tumors, leukemia cells and tissues are compatible with the notion that the effective blocking of the three main putrescine sources (intracellular decarboxylation of ornithine, formation of putrescine from N1-acetylspermidine, and the gastrointestinal tract) produces a very strong cytostatic effect. It is expected that the clinical efficacy of polyamine antimetabolites can be considerably improved by measures analogous to those applied in this pilot study.