Objectives– The etiology of MS is unknown but genetic factors are supported by a high concordance in twins. Geographic distribution and migration studies indicate, however, the importance of environmental factors. Material and methods– We studied 3 pairs of genetically identical twins who had shared the same environment but were discordant for MS. Serum samples were assayed for antibodies against 21 viruses, 4 bacteria and Toxoplasma gondii. Results and conclusion– No common factor present only in the affected twins was identified but differences were found in serum titers against some neurotropic microorganisms. In general the serum titers were strikingly similar in the twins, indicating no major disturbances of the humoral immune system in MS.