An almost complete prevention of tumor growth was achieved in U-251 human glioblastoma xenografted nude mice, by partial decontamination of the gastrointestinal tract and feeding of a polyamine-free diet containing inhibitors of ornithine decarboxylase (DFMO) and of polyamine oxidase (MDL 72527). After one week of polyamine deprivation, spermidine concentrations were lowered, and spermine levels were increased in all tissues. In contrast, putrescine concentrations were only reduced in tumor and in brain. Erythrocyte polyamine determinations revealed differences similar to those observed in tissues: spermidine concentration was lowered by 50% and spermine level was 3-fold increased. If this or related treatments should become of therapeutic importance in the future, then the determination of erythrocyte polyamine levels might be of diagnostic value.