We studied infectious and immune mechanisms in demyelinating disease. The clinical diagnosis in this study of 313 consecutive cases of multiple sclerosis (MS) was based on the clinical conclusions of two or more neurologists and definite abnormalities in CSF IgG. Measurement of antibodies to six microbial agents was compared in 313 patients with MS and 406 controls in the same age range. Using a standardized immunofluorescent antibody (IFA) technique, we found a significantly higher prevalence of Epstein-Barr virus (EBV) infection and a higher level of serum viral capsid antigen IgG antibody titer in the MS population than in the controls. The MS population had a lower cytomegalovirus (CMV) infection rate and lower CMV complement fixing antibody production than controls. Except for the higher measles infection rate and antibody titer in patients with MS, data on the other viruses did not differ from controls.