Abnormalities were detected by two-dimensional gel electrophoresis in the protein composition of both the dorsal and ventral roots of three of six patients who succumbed to amyotrophic lateral sclerosis (ALS). The abnormalities consisted of a cascade of acidic protein spots on silver-stained gels which were shown by immunoblotting to react with an antiserum to human glial fibrillary acidic protein (GFAP). They were found distal to the normal central nervous system/peripheral nervous system (CNS/PNS) transition zone and were undetected in cervical and lumbar root segments taken at the same distances from the spinal cord of eight control patients. Similar changes were observed in the dorsal and ventral roots of one patient with Werdnig-Hoffmann disease (WHD), while a second patient with WHD had the changes in only the ventral roots. The abnormalities probably reflect the presence of radicular glial bundles, which are pathological extensions of glial cells into the spinal roots, indicating that subclinical changes occurred in the sensory nerves of the affected ALS and WHD patients. While no other qualitative abnormalities were noted on gels of ALS and WHD spinal roots, some quantitative changes may be present.