Well-preserved nuclei were isolated from the brain of a patient with neuronal intranuclear inclusion disease ( NIID ). The inclusions in situ stained for proteins and, by immunohistochemical techniques, for the 200K and 68K components of the neurofilament protein triplet but were negative for the 145K neurofilament protein. At the electron microscopic level the inclusions were seen to consist of straight, randomly arranged tubular filaments 8.5 to 9.5 nm in diameter. They were resistant to extraction with hot buffer containing sodium dodecyl sulfate (SDS) and 2-mercaptoethanol, indicating that the proteins of NIID inclusions are probably cross-linked by covalent bonds other than disulfide. This feature is also exhibited by the paired helical filaments occurring in brain in Alzheimer's disease. No major differences were found in the polypeptide composition of nuclei in NIID and control nuclei resolved by SDS-polyacrylamide gel electrophoresis. The inclusions isolated from SDS-extracted nuclei were shown by immunohistochemical techniques to stain for all three neurofilament subunits, indicating that 145K -related antigenic sites were uncovered during the extraction procedure.