The nature of Lewy bodies (LBs) in the brain stem and cerebral cortex in five cases of diffuse Lewy body disease and one case of Parkinson's disease with dementia were investigated immunocytochemically with various antibodies to cytoskeletal proteins, paired helical filaments (PHF) and ubiquitin. Antibodies to 200-kDa component of neurofilament, tau and PHF showed no significant reactions with most of LBs. Antibodies to high-molecular weight microtubule-associated proteins (HMWMAPs) moderately stained the periphery of a few of LBs. A monoclonal antibody to PHF (DF2) which recognizes ubiquitin, and polyclonal antibodies to ubiquitin immunostained virtually all of the typical and cortical LBs as intensely as Alzheimer's neurofibrillary tangles and senile plaque neurites: the periphery of LBs was darkly stained, whereas the central core of typical LBs and central zone of cortical LBs were less intensely stained or remained unstained. Immunoelectron microscopy of the LBs with DF2 revealed that immune reaction products were located on the filaments exclusively in the periphery of LBs, but not on those in the center. These findings suggest that both types of LBs are immunocytochemically indistinguishable despite some structural differences, and that peripherally located filaments in LBs are tagged with ubiquitin, an element required for the ATP-dependent proteolysis system in the cell. Antibodies to ubiquitin are the most useful marker of LBs ever known.