Extremely severe gliosis develops at the end stage of infantile neuronal ceroid-lipofuscinosis (INCL), a fatal encephalopathy characterized by accumulation of autofluorescent storage material in the brain and other tissues followed by a terminal subtotal neuronal and myelin loss. A major fraction of highly enriched intermediate filaments was obtained with a density gradient centrifugation method from INCL brain tissue, whereas the storage material represented only a minor fraction. SDS-polyacrylamide gel electrophoresis of the filament fraction showed a major protein with molecular weight of 51 kD and three to four polypeptides of 40–48 kD identified as glial fibrillary acidic protein (GFAP) and its degradation products by the immunoblotting technique with monoclonal antibodies against GFAP. Immunization experiments with the isolated INCL glial filament fraction produced antibodies reacting only with GFAP but not with other types of intermediate filament proteins, furthermore indicating a high content of GFAP in the isolated fraction. No significant amounts of vimentin or other types of intermediate filament proteins could be detected. These results document the extremely high content of glial filaments at the terminal stage of INCL and suggest that INCL brain may serve as a good human model for studies on the composition of glial filaments in vivo and on the pathogenesis of gliosis.