Astrocytes are pivotal components of immune reactions in the CNS. We further support this notion by the localization of the lymphokine interferon-gamma (IFN-gamma), which plays an important role during immune responses, to astrocytes in rat optic nerve (ON). Astrocytes identified by glial fibrillary acidic protein immunoreactivity were IFN-gamma positive in normal and transected ON while oligodendrocytes did not express IFN-gamma immunoreactivity. These findings indicate that astrocytes can generate important signals which orchestrate immunoinflammatory responses in the brain.