Activity of the systems involved in generation and utilization of superoxide radicals was studied in microsomes, mitochondria and nuclei of liver tissue from intact mice, mice with developed Ehrlich ascites carcinoma and of the animals treated with antitumoral drug ruboxyl. The ratio between the rate of superoxide radicals formation and activity of superoxide dismutase (SOD) served as specific characteristic of the O2(-)-SOD system in the corresponding compartments. During tumoral development, the pattern studied was altered in all the subcellular organelles used, thus demonstrating an impairment of free radical oxidation status in liver tissue of tumor-bearing animals. Administration of ruboxyl into healthy animals led to distinct increase in this ratio in mitochondria, while the drug normalized patterns of the O-(-)-SOD system in all the cell compartments studied in tumor-bearing animals. Ruboxyl appears to exhibit regulating effect on free radical oxidation.