The frequency of the MLC activating HLA-Dw2 determinant was found significantly increased to 68% among 37 patients with long-standing multiple sclerosis, compared to 30.2% among healthy controls. A similar (or stronger) degree of association was found with an "Ia-like" B cell specificity, SOW, while the association with HLA-B7 was less apparent. No correlation between HLA-Dw2 and signs of local synthesis of oligoclonal IgG or measles or rubella virus antibodies in the CNS was observed.