Antioxidant enzymes (AOE) may be considered as possible regulatory factors of tumor cell proliferation. Enzyme levels were examined in Ehrlich's carcinoma in ascitic and solid forms differed in proliferative rate. Antioxidant enzyme activities depended on cancer tissue organization. Studies of enzyme activities at different phases of tumor growth showed reduction of superoxide dismutase, glutathione peroxidase and glutathione transferase activities at the logarithmic phase and an increase of the activities at the confluent phase. These data confirm the suggestion about a rise in intracellular content of oxygen radicals at logarithmic growth and possibility of active oxygen metabolite involvement in the regulation of cell proliferation.