Viral antibodies to measles, rubella, corona, vaccinia, and mumps viruses in serum and CSF (and to Epstein-Barr virus in serum only) were studied in 24 twin pairs, both discordant and concordant for clinical MS. In pairs, CSF antibody titers for rubella in MS monozygotic and dizygotic twins and for vaccinia in dizygotic twins were higher than for unaffected twins. Increased CSF titers among MS twins existed for measles, rubella, and vaccinia when pairing was ignored. Among MS twins, serum rubella and measles and CSF measles antibody titers, and CSF:serum ratios for measles virus, were higher in those who were DW2 positive.