Glutathione peroxidase (GSH-Px) activity, one of the scavenger enzymes of oxygen active radicals, has been measured in hepatocellular carcinoma (HCC) of 17 patients and the values compared with the activity of adjacent tumor-free tissue and with those of 30 histologically normal livers. The results demonstrate a reduced GSH-Px activity in neoplastic tissue (21.19 vs 33.74 U/g prot.; P less than 0.001). However, the adjacent tumor-free liver also had a reduced activity when compared with normal tissue (23.15 vs 33.74 U/g prot.; P less than 0.01), but this value did not differ from that of HCC tissue. These data suggest that HCC might develop in a GSH-Px-deficient condition.