This is the first report which demonstrates the presence of glutathione peroxidase in the autopsied brain of 5 patients without cerebral infarction and 21 patients with cerebral infarction by the indirect enzyme-labeled antibody technique with monoclonal antibody to human glutathione peroxidase. In 2 out of 5 patients without cerebral infarction, a weak reaction for glutathione peroxidase was demonstrated both in neurons and glia. In 6 patients who had died within 5 days after stroke, no staining was observed in infarcted brain tissue except in macrophages. In all 15 patients who had died more than 6 days after stroke, however, a reaction for glutathione peroxidase was demonstrated in the cytoplasm of glial cells in the marginal area around the infarction, and there was a patchy reaction in the cytoplasm of macrophages in the core lesion. These results suggest that glutathione peroxidase in glial cells of the marginal area around the infarction may play a protective role against lipid peroxidation after cerebral infarction, or alternatively, may be involved in the healing process after ischemia.