Selenium-dependent (Se-GSH-Px), selenium-independent (non-Se-GSH-Px) glutathione peroxidase and glutathione reductase (GSSG-Rx) activities have been determined in cancerous and non-cancerous human adult kidney. Large inter-individual variation in the activities of all enzymes tested were found in both tumour and non-tumour specimens. In general a significant decrease in the activities of the three enzymes was found in tumours. When a comparison was made between cancerous and non-cancerous tissues of the same individual, Se-GSH-Px activity was found to be lower in tumour in 17 cases out of 29, and the non-Se-GSH-Px activity in 20. In 20 cases out of 29 GSSG-Rx was found to be lower in tumour. It was concluded that changes in the factors involved in the anti-oxidative protection actually occur in human kidney tumour.