By a variety of assays, investigators (1-4) from several laboratories previously observed changes in numbers of thymus-derived suppressor lymphocytes (Ts) in patients with acute active multiple sclerosis (MS). Changes in numbers of Ts in the peripheral blood compartment of such patients closely follow disease activity. Extending these observations we now report Ts are activated in the peripheral blood of certain patients with MS. This activation occurred in vivo in the majority of patients with active disease, but only during remission after an acute episode of MS. In contrast, activated Ts were found infrequently in patients with chronic progressive or stable MS, with neurologic diseases other than MS, or healthy individuals. The suppressor activity found in peripheral blood T cells during the acute remitting stage of disease was associated with elevated numbers of lymphocytes bearing Fc receptors for IgG and/or OKT8 markers.