Glutathione peroxidase was assayed in human tissues of New Zealand residents by the coupled assay method. Total glutathione peroxidase was assayed using cumene hydroperoxide. The non-selenium-dependent activity was not detected with t-butyl hydroperoxide and thus was determined from the difference between total activity and the selenium-dependent activity using hydrogen peroxide or t-butyl hydroperoxide. Only selenium-dependent activity was found in whole blood, erythrocytes, platelets and biopsy skeletal muscle. A small non-selenium dependent activity was measured in plasma and a larger activity in biopsy liver supernatant and homogenate. Glutathione-S-transferase was detected in all tissues.