The activities of superoxide dismutases (total, cytoplasmic and mitochondrial) and glutathione peroxidase were measured in 10 cancerous and 10 non-cancerous adjacent human kidney tissues. Total (T-SOD) and cytoplasmic (Cu, Zn-SOD) superoxide dismutase and glutathione peroxidase (GSH-Px) activities were found lower in cancerous tissues compared with those of non- cancerous ones. However, no difference was found between the mitochondrial (Mn-SOD) superoxide dismutase activities of the tissues. Similarly, no differences were observed between the enzyme activity values of the tissues at stages I-II and III-IV renal cancer. In correlation analysis the positive relation found between Cu, Zn-SOD and GSH-Px enzymes in the non-cancerous tissues was found to be absent in the cancerous ones. The results suggest that enzymatic free radical defense mechanism is significantly reduced in the cancerous human kidney tissues due to depressed Cu, Zn-SOD and GSH-Px activities.