Although many studies support a role for viruses in multiple sclerosis (MS) etiopathology, no specific agent has been consistently associated with significant numbers of MS patients without concomitant detection in non-MS controls. Previous studies have shown the presence of viral-like structures in MS plaques, although the specificity of these structures for MS has been questioned. The present study describes the use of polyclonal antisera against feline and human brain-derived cytopathic agents and immunoaffinity chromatography to purify and partially characterize possible virus-like structures from MS brain tissue. Chromatography eluates from 4 MS brains contained pleomorphic particles up to 350 nm in diameter and tubular/filamentous-like structures approximately 10-18 nm in thickness. Inoculation of primary rat glial cell cultures with chromatography eluates from MS brain tissue resulted in a reproducible pattern of cytopathic effects in the form of multinucleation in cells identified immunocytochemically as oligodendrocytes. Antisera raised against the feline and MS-derived cytopathic agents were used to successfully immunolabel infected oligodendrocyte-like cells and syncytia and to detect a 66,000 M(r) protein on Western blots of inoculated cultures or concentrated MS brain eluates. Similar structures, cytopathic effects (CPE) and protein expression were not observed in eluates from 5 control brains or in cultures inoculated with control brain eluates. These studies demonstrate that cytopathic, virus-like structures can be isolated from MS brain tissue using antisera raised against a cytopathic agent rescued from demyelinating brain lesions in cats. The identity of this agent and its possible role in MS aetio-pathology remains unknown.