We demonstrated antibodies to isolated oligodendrocytes and to oligodendroglia in brain sections by indirect immunofluorescence technic in serums of 19 of 21 patients with multiple sclerosis. We also found such antibodies in three of five patients with subacute sclerosing panencephalitis and one of four patients with acute disseminated encephalomyelitis, but not in patients with other neurologic diseases or normal persons. The antibodies were absorbed by preincubation of serum with isolated oligodendrocytes or whole white matter, but not with purified myelin or liver tissue. Immunofluorescent staining was blocked by either rabbit anti-oligodendrocyte serum or non-fluoresceinated goat anti-human immunoglobulin. These findings suggest that antibodies to oligodendroglia are distinct from antibodies to myelin and that demyelination in multiple sclerosis could be a consequence of an immunopathologic reaction directed against oligodendroglial cells.