The factors responsible for the major demyelinating disease of the central nervous system (CNS), multiple sclerosis, are poorly defined. Although T-cell-mediated immune responses play a pivotal role in establishing the inflammatory response, humoral factors also may be critical in disease progress. We have isolated a mouse monoclonal antibody (mAb 2B10) that recognizes a cell-surface molecule expressed exclusively by rat oligodendrocytes, the cells responsible for the formation and maintenance of CNS myelin. In cultures of neonatal rat spinal cord, mAb 2B10 specifically mediated oligodendrocyte cell death in the absence of complement. In the current study, mAb 2B10-producing hybridoma cells were implanted into adult rat brain ventricles, and the effect of mAb 2B10 on CNS cytoarchitecture was examined. In the optic nerves of mAb 2B10-treated animals, there was significant focal myelin degeneration near the optic chiasm. Axons in the myelin degenerate regions were largely healthy. There was no significant infiltration of hematopoietic-derived cells into the affected regions, but microglia were activated focally and phagocytosed the collapsed myelin. This study demonstrates that an antibody directed against myelin-forming cells induces CNS demyelination and supports the hypothesis that autoantibodies may play a role in CNS demyelinating diseases.