In 36 patients representing different clinical stages of multiple sclerosis (MS) (9 patients with acute exacerbations; 21 patients in remission; 5 patients with chronic progressive MS) determinations of T lymphocyte populations using monoclonal antibodies against surface antigens (OKT3 (pan T cells), OKT4 (helper T cells), OKT8 (cytotoxic/suppressor T cells] were performed. Compared to the control group (40 healthy individuals) a clear elevation of the T4/T8 ratio was found in acute exacerbations and to a lesser degree in patients with inactive phases of MS. Patients with chronic progressive disease did not show increased T4/T8 ratios. Serial determination of lymphocyte populations after corticosteroid therapy in 10 selected patients revealed no significant changes which could be attributed to this therapeutic modality. Pathogenetic and clinical implications of the shifts in surface antigen expression of T lymphocyte populations mirroring the clinical course of MS are discussed.