Bone-marrow aspirates from 5 patients with recent episodes of multiple sclerosis (M.S.) contained increased numbers of plasma cells, and aspirates from 3 of these patients contained atypical reticulum cells. When bone-marrow from 4 of the patients was inoculated into cell cultures a cytopathic effect (C.P.E.) was observed, and this effect could be serially passaged to further cultures. The C.P.E. was not observed when ether-treated extracts of cells showing C.P.E. were used for passage or when the passage material was filtered through a 100 nm filter. Appearance of C.P.E. was delayed in cultures inoculated with 220 nm filtrates of inoculum was passed through filters of greater pore size. No C.P.E. was observed after culture of bone-marrow aspirated from each of 7 patients under clinical investigation for disorders other than M.S.