Serum viral antibody titers against 21 viruses were studied in 19 of 23 same-sex twin pairs with multiple sclerosis derived from the Finnish Twin Cohort. Thorough neurologic examinations showed two monozygotic pairs to be concordant, whereas all dizygotic pairs were discordant. Special attention was given to measles, mumps, and rubella viruses, against which the antibody levels were determined with the complement fixation, hemagglutination inhibition, hemolysis-ingel, and enzyme immunoassay methods. Epstein-Barr virus antibody levels were determined by enzyme assay. In pairwise comparisons, the measles, mumps, and Epstein-Barr virus-IgG antibody levels were more often elevated in the patients with multiple sclerosis, compared with the healthy co-twins. The same antibody levels were more often above the median in the diseased twin, compared with the healthy twin, but the difference was not significant. No human T-cell lymphotropic virus type I antibodies were found in any of the individuals examined. The total IgG, IgA, and IgM levels did not differ between the diseased and healthy subjects. The HLA types, severity of the disease, and cell-mediated immunity parameters did not influence antibody levels.