Since there is no satisfactory animal model for multiple sclerosis (MS), a need exists to study various tissues of the affected patient himself. Leukapheresis is a relatively non-invasive technique for obtaining large numbers of leucocytes from patients and control patients. Using ACD-B solution as anticoagulant, the procedure can process 3 500 ml. of blood in about 2 hours, and yield an average of 6.7 billion lymphocytes. The circulating blood count and differential leucocyte count are not changed significantly at the end of the procedure, although the platelet count drops slightly. Leukapheresis has been performed 6 times in four MS patients representing all stages of the illness. The data indicate that the procedure, as performed, is safe in MS patients and do not adversely alter the course of the illness. Leukapheresis is not suggested for therapy at this time. However, if further study suggests that circulating lymphocytes in the MS patient have a cytolytic function in the central nervous system (CNS), the procedure might merit a trial in a controlled treatment program. At present, leukapheresis will probably be most useful as a research tool, to aid study of the role of the lymphocyte in the pathogenesis of MS.