We have studied the response and the spatial distribution pattern of microglial cells during experimental allergic neuritis induced in the Lewis rat by the transfer of varying doses of activated T cells specific either for the P2 or P0 protein. The microglial reaction was studied immunocytochemically at the light and electron microscopic level using a panel of monoclonal antibodies which included two recently produced antibodies against rat microglial cells, Murine Clone 101 and 102. Activation of microglial cells became apparent through changes in their immunophenotype and morphology within 48 hours of T cell transfer and therefore preceded the onset of clinical disease. Activated microglial cells showed an increased expression of the complement type three receptor, the murine clone 101 and 102 determinants and major histocompatibility complex antigens. The microglial reaction in experimental allergic neuritis occurs at a site remote from the inflammatory changes in the peripheral nerve, the microglial reaction being most prominent in the dorsal and ventral grey matter of the lumbar and the thoracic spinal cord. Similar changes were also observed at this time in the terminal projection fields of the primary, afferent, sensory fibers, such as the nucleus gracilis. Subsequently, after 7 days, motoneurons, particularly in the ventral grey matter of the lumbar spinal cord, were ensheathed by perineuronal microglial cells. These perineuronal microglial cells were in close contact with the neuronal plasma membrane and occasionally appeared to detach afferent synaptic terminals from the surface. Microglial responses were not detected in animals injected with nonpathogenic T cells specific either for the purified protein derivative or ovalbumin. This early activation of microglial cells observed in experimental allergic neuritis suggests that a rapid and remote signaling might be operating in the microglial responses during T cell-mediated autoimmune diseases.