There is indirect evidence that key enzymes which catalyse both synthesis and degradation of polyamines (PA) may be useful as biological markers of malignant growth. Ornithine decarboxylase (ODC) activity was assessed in fetal, regenerating and adult rat liver, in hepatoma with different growth rate, in Morris hepatoma cell lines and in rat liver in the course of nitrosopiperidine (NP) carcinogenesis. A much higher ODC activity was observed in all the investigated tumor tissues and in regenerating rat liver as well. A significant decrease of ODC inductive synthesis was demonstrated both in liver and hepatoma of rats maintained on vitamin B6-deficient diet. The experimental suggest that inductive synthesis of ODC may be considered not only early, but also the mandatory event in any hyperproliferating processes including carcinogenesis.