A longitudinal study on rubella, measles, and respiratory syncytial virus antibodies in serial serum and CSF specimens from 20 multiple sclerosis (MS) patients was performed, using solid-phase radioimmunoassay. Albumin and immunoglobulin G (IgG) levels were also measured to check the integrity of the blood-brain barrier and the intrathecal IgG production. All the patients had local IgG production in their CNA. A local antibody production against one or more of the viruses studied was evident in 15 patients. Fluctuations in the intrathecal viral antibody synthesis were evident in eight patients. No correlation was found between these changes and the clinical course of the disease. The results suggest that the intrathecal antibody synthesis in MS is only partially against any given virus, and in most patients the bulk of the oligoclonal CSF antibodies is against antigens other than those studied here.