We measured activities of superoxide dismutase, catalase, and glutathione peroxidase and content of nonprotein thiol groups (reduced glutathione) in blood lymphocytes from patients with rheumatoid arthritis before and during bioresonance therapy. The state of the antioxidant system in lymphocyte from patients receiving standard pharmacotherapy was characterized by activation of the key antioxidant enzymes and decreased content of thiol groups. Bioresonance therapy increased the content of thiol groups and normalized activities of superoxide dismutase and glutathione peroxidase. However, catalase activity remained above the control. Changes in the lymphocyte antioxidant system indicate that bioresonance therapy activates nonspecific protective mechanisms in patients with rheumatoid arthritis.