Recently it has been shown by light microscopic immunochemistry that Lewy bodies (LBs) react with antibodies raised against neurofilament proteins (NFPs). Because of the ubiquity of the NFPs within neurons, the heterogeneous makeup of the inclusions, and the varying patterns of immunolabeling, we undertook to determine whether the labeled elements are indeed constituent filaments. Employing a preembedding technique, we investigated sections of the same LBs by light and electron immunochemistry. Decoration of the filaments was obtained with a monoclonal anti-NFP antibody. Whereas cores of mature LBs were unreactive by light microscopy, these same cores yielded a positive reaction at the ultrastructural level. Early LBs were intensely labeled in both the core and periphery. These results demonstrate that the filamentous profiles that form the LBs are antigenically identical to neurofilaments and suggest a posttranslational modification of the filaments as they "age" within the inclusion.