6 B-lymphocyte alloantigens have been provisionally identified with lymphocytotoxic antisera reacting, after absorption, specifically with B but not T cells. 3 of these antigens appear to form part of an allelic series. The frequency of HLA and B-lymphocyte antigens was then studied in 59 patients with multiple sclerosis (M.S.). 1 of the B-cell antigens, BT 101, was found in 49 out of 59 patients (83%), compared with 10 out of 30 normal individuals (33%), giving a relative risk of 9-8 times to the association. 2 other B-cell alloantigens and HLA-B7 showed lesser but significant positive associations with M.S. Apart from providing possible clues to the pathogenesis of M.S., the association between BT 101 and M.S. may allow screening for susceptible individuals who are thought to be at special risk.