Reactive astrogliosis has been found in the cerebral cortex and spinal cord in amyotrophic lateral sclerosis, whereas astrocytic pathology in immune-mediated motor neuron injury has not been reported. On the basis of the establishment of an immune-mediated motor neuron injury model, histological, immunohistochemical and ultrastructural studies on astrocytes were carried out. Reactive astrogliosis in spinal cord anterior horn was observed, and the increase of GFAP-positive astrocytes may occur prior to the clinical manifestations. Reactive astrocytes could show pathological changes similar to those of motor neurons, such as swollen mitochondria and dilated endoplasmic reticulum. Of special interest are crystallized inclusions that were frequently observed in the reactive astrocytes in the bovine ventral horn homogenate-immunized anterior horn. The precise nature and origin of these inclusions deserve to be elucidated.