Specific antibody activities (antibody per weight unit IgG) of serum and CSF against a broad variety of viruses were compared in multiple sclerosis and certain inflammatory diseases of the central nervous system, e.g. neurosyphilis, as well as herpes simplex and zoster encephalitis. No "unspecific" antiviral activities within the CSF compartment were found in the non-MS diseases. The most frequent antibodies locally produced were directed against measles, rubella and zoster antigens. A diagnostic test with these three viruses would give positive results in about 80% of patients with MS. This finding is not as frequent as the oligoclonal pattern of the CSF gamma-globulins but would have a considerably greater diagnostic significance.