Spontaneous mouse liver chemiluminescence (109 +/- 6 cps/cm2) was increased in the early phase after tumor implantation in a distant position with respect to the liver. A 39% increased liver chemiluminescence was observed after 5 days of the injection of Ehrlich ascites tumor cells into the peritoneal cavity, and a 64% and a 46% increased liver chemiluminescence were measured after 8 and 14 days of the implantation of a fibrosarcoma and of an adenocarcinoma, respectively, in the leg. At the time of maximal stimulation of in vivo liver chemiluminescence by the distant tumors, cytosolic superoxide dismutase, catalase, and glutathione peroxidase activities were decreased by 18%, 38%, and 26% in the liver of mice bearing Ehrlich ascites tumors. The same three enzymatic activities were decreased by 21%, 19%, and 54% respectively, in the liver of fibrosarcoma-bearing mice. Total liver glutathione was decreased by 18% to 22% in the tumor-bearing animals. Hydroperoxide-initiated chemiluminescence was increased in the homogenates (105% and 45%) and mitochondria (64% and 34%) from the liver of mice bearing Ehrlich ascites tumors and fibrosarcomas, respectively, at the time of maximal in situ liver chemiluminescence. The hydroperoxide-initiated chemiluminescence of liver microsomes was decreased by 46% to 36% in the tumor-bearing animals at the same time. It is concluded that the liver of tumor-bearing animals is subjected, during the early phase after tumor implantation, to an oxidative stress with increased steady-state levels of peroxyl radicals, which are essentially responsible for the increased photoemission observed in vivo.