• Twenty of 41 patients with multiple sclerosis (MS) and five of 30 patients with other neurological diseases had measles antibody titers in their CSF when examined in hemagglutination inhibition and neutralization tests. Quantitation of measles-specific IgG in CSF of patients with MS with significant measles antibody titers showed that less than 5% of the total IgG was measles virus specific. In contrast, using the identical quantitation procedures, 30% to 60% of the total IgG in CSF of patients with subacute sclerosing panencephalitis was found to be measles specific. Thus, only a very minor portion of the total IgG in CSF of patients with MS is synthesized against measles virus. When measles hemagglutination inhibition, complement fixation, and neutralizing antibody titers were measured in individual oligoclonal IgG bands isolated from neutral pH brain extracts of two patients with MS, low titers were present in all the oligoclonal bands rather than being restricted to any single band.