Measles virus‐specific antibodies were isolated from sera, cerebrospinal fluids (CSF), and brain extracts of patients with subacute sclerosing panencephalitis (SSPE) and multiple sclerosis (MS) by absorption with measles antigens and subsequent acid elution of the antigen–antibody precipitates. Electrophoretically homogeneous measles antibodies were isolated from CSF or brain extracts in five patients with SSPE and in five out of seven patients with MS. Homogeneous IgG antibodies were also demonstrated in the sera from all SSPE patients and from three of the MS patients. The antibodies isolated from various control sera and from pooled CSF were electrophoretically heterogeneous. The results support the concept of a local synthesis in the nervous system of oligoclonal IgG antibodies to measles virus in all patients with SSPH and in some patients with MS. In SSPE, most or all oligoclonal IgG proteins of the CSF or brain carry measles antibody activities. In MS, only part of the oligoclonal IgG appears to be associated with measles antibody activity