Mitogen responsiveness and suppressor cell function were determined in patients with multiple sclerosis. Mitogen response to concanavalin A, expressed as stimulation index, decreased with age in both patients and controls. Although responses in the overall group of young patients did not differ from controls, the stimulation index was reduced in clinically inactive patients. Patients with recent flare-ups showed responses comparable to controls. Suppressor cell activity increased with age in both disease and control groups. Among young patients, suppressor activity was borderline low in inactive patients but increased significantly in those with recent exacerbations.