B-lymphoblastoid cell-lines may develop spontaneously in mononuclear cells from patients with multiple sclerosis, an observation rarely seen in healthy individuals. Examination of such spontaneously established B-cell lines reveal the presence of Epstein-Barr virus and retrovirus particles. We have speculated that in predisposed individuals, a dual infection with retrovirus and late acquired Epstein-Barr virus plays an aetiological role in the development of multiple sclerosis. This hypothesis is supported by a number of observations, including the finding that infection with Epstein-Barr virus may be a prerequisite for developing multiple sclerosis. The association between multiple sclerosis and infection with Epstein-Barr virus and retrovirus is evaluated in this study.