Bunina bodies (BBs), small eosinophilic intraneuronal inclusions in the remaining lower motor neurons, are the only pathologically specific hallmark of amyotrophic lateral sclerosis (ALS). During immunohistochemical examinations of spinal cords of patients with ALS, we noted that BBs were positive for anti-cystatin C (CC) serum. Immunoelectron microscopically, many small deposits of immunoperoxidase products were seen in the cytoplasms and dendrites of the anterior horn cells. Some immunoperoxidase products exhibited a tubular or vesicular pattern, but no rough endoplasmic reticula, mitochondria, lipofuscin granules or nuclei were detected. The presence of BBs, especially in the periphery, was marked. In an adult cat, CC was mainly localized in the medial aspects of Golgi apparatus and in lysosomes in the anterior horn cells of the spinal cord. BBs may represent an abnormal accumulation of unknown proteinous material associated with the Golgi apparatus.