The pathogenesis of chronic manifestations in Lyme borreliosis, a disease induced by Borrelia burgdorferi, is at present unresolved. By testing monoclonal antibodies directed against various borrelia antigens, we found an antigenic determinant shared by the 41 kDa flagella protein and human tissue, especially prominent on myelinated fibers of human peripheral nerve, on nerve cells and axons of the central nervous system, as well as on certain epithelial cells (including joint synovia) and on heart muscle cells. Immune reactions against such a shared antigen could play a pathogenetic role in chronic organ manifestations of Lyme borreliosis.