Plasma total peroxyl radical scavenging capacity was studied in terminal patients who were chronically immobilized because of an acute (stroke) or chronic neurodegenerative disease (Alzheimer's disease). A luminometric assay was used to measure total antioxidant capacity (TRAP). The immobilized patients showed significant decrease in TRAP primarily because of a decrease in the concentration of unknown antioxidants. Our results suggest that human plasma may contain unknown antioxidants, the regulation of which could be related to the basic metabolic rate.