The molecular mechanisms underlying myelin sheath destruction in multiple sclerosis lesions remain unresolved. With immunogold–labeled peptides of myelin antigens and high–resolution microscopy, techniques that can detect antigen–specific antibodies in situ, we have identified autoantibodies specific for the central nervous system myelin antigen myelin/oligodendrocyte glycoprotein. These autoantibodies were specifically bound to disintegrating myelin around axons in lesions of acute multiple sclerosis and the marmoset model of allergic encephalomyelitis. These findings represent direct evidence that autoantibodies against a specific myelin protein mediate target membrane damage in central nervous system demyelinating disease.