T, B, and “null” lymphocytes were determined in peripheral blood obtained from 24 control subjects and 15 multiple sclerosis patients. The total number of lymphocytes and T lymphocytes were reduced significantly in multiple sclerosis patients as compared with those of controls. Although the percentage of B lymphocytes were increased in these patients, the absolute number of B lymphocytes was similar to controls. The mitotic response of lymphocytes from multiple sclerosis patients to phytohemagglutinin and pokeweed mitogen in 7-day-old cultures was decreased as compared with that of controls.