Immunocytochemical and quantitative immunochemical techniques were used to study the expression of Alz-50 antigen, ubiquitin and Tau in neurologic disorders characterized by the formation of filamentous neuronal inclusions. Alz-50, anti-ubiquitin and Tau-1 immunostained the intraneuronal neurofibrillary tangles and the neuritic component of plaques, both in Alzheimer's disease and in the brains of patients without dementia, but extraneuronal tangles were largely unstained. These antibodies also reacted with Pick bodies, and with the neurofibrillary tangles of Kufs' disease and Guam Parkinsonism-dementia. In sections from the brain of a patient with progressive supranuclear palsy, virtually all of the tangles were immunostained with Tau-1 but only a few with Alz-50 or anti-ubiquitin. Anti-ubiquitin also labelled Lewy bodies and the inclusions of granulovacuolar degeneration. Quantitative analysis of immunoblots of homogenized frontal cortex showed significantly more Alz-50 antigen in the brains of patients with Alzheimer's and Pick's disease than in controls. The level of this antigen was increased both in the crude homogenates and in the cytosolic fraction. Ubiquitin immunoreactivity was increased only in the brains of patients with Alzheimer's disease and then only in the crude homogenates. The finding that antigenic determinants for Alz-50, anti-ubiquitin and Tau-1 are shared by several filamentous neuronal inclusions occurring in diverse neurologic disorders may reflect common metabolic defects underlying the formation of these inclusions, or common metabolic responses to their presence.