A monoclonal antibody to p24 of human immunodeficiency virus (HIV) was demonstrated to react with degenerated neurons immunohistochemically. In a case of Pick's disease, the neurons of the dentate gyrus were labeled with the antibody, while normal-looking ones and glial components were negative. The positive structures showed argyrophilia by the Bielschowsky and Bodian methods. Antibodies to ubiquitin, paired helical filament and tau did not react with the inclusions. Ultrastructurally, they consisted of skeins of fuzzy-surfaced fibrils with a diameter of 15 nm. In cases of Alzheimer type dementia, thread-like positive profiles were observed in some neurofibrillary tangle-bearing neurons. These studies revealed the cross-reactivity of the anti-HIV p24 antibody to unknown types of neuronal inclusions and provide a new aspect for research into neurodegenerative disorders.