Immunoprecipitation methods were used to analyze sera and cerebrospinal fluids from patients with multiple sclerosis for measles virus-specific antibodies. The sera contained antibodies against all virus antigens except the matrix polypeptide. Of the nine cerebrospinal fluids investigated, two contained undetectable levels of measles-specific antibodies, four contained antibodies against the nucleocapsid and another internal antigen, and three contained antibodies against the surface antigens as well as the internal antigens.