Detailed comparative ultrastructural examination of multiple sclerosis (MS) plaques, inflammatory CNS lesions from adreno-leukodystrophy(A-LD), tissue from a case of chronic granulomatous meningitis, biopsy samples of necrotic cerebral cortex and CNS tissue from a case of Kuf's disease (adult-type ceroid lipofuscinosis), has revealed that the intranuclear filamentous material previously thought to be related to a viral infection in MS is a non-specific finding. These intranuclear strands were, however, found in greatest frequency in the acute lesions of MS and were absent from chronically demyelinated areas. The macrophages, lymphocytes and fibrocytes containing filamentous material in the nuclei were mainly perivascular. In A-LD, some macrophages in active lesions contained similar nuclei, and in Kuf's disease they were present in some glial cells in the cerebral cortex.