Most of the identified constituents of the filamentous inclusions characteristic of the neurodegenerative diseases of aging are derived from the cytoskeleton. This study was undertaken to define immunocytochemically the cytoskeletal constituents of the filamentous cytopathologic marker of idiopathic Parkinson disease, the Lewy body (LB). An array of antibodies specific to neurofilaments, tubulin, microtubule associated proteins (tau, MAP1 and MAP2) and Alzheimer neurofibrillary tangles (NFT) were used to immunostain sections containing LB. All the antibodies to tubulin, MAP1 and MAP2 and the majority of the antibodies to neurofilaments and NFT recognized LB. The two monoclonal antibodies to NFT that recognize LB also react with ubiquitin, which has been identified in NFT. The prominent NFT component, tau, is apparently not incorporated into LB. These findings suggest that the presence of tau might not be a prerequisite to the formation of abnormal filaments. Therefore, although LB contain elements of neurofilaments, microtubules and ubiquitin, as do other abnormal neuronal filaments, they are distinct in composition. These distinctive and shared features may provide useful insights regarding the mechanisms underlying the formation of filaments in LB as well as those of other neuronal inclusions.