Abstract An immunological mechanism may be responsible for the peripheral neuropathies associated with the IgM monoclonal gammopathies. Myelin-associated glycoprotein, and sulfated glucuronic acid-glycosphingolipids of the paragloboside type specific of peripheral nervous system (PNS), have been found to be targets for these antibodies. We report here on the presence of a glycolipid antigen present in central nervous system white matter in 7 of 12 patients with IgM monoclonal gammopathy and neuropathy whose IgM reacts also with the specific sulfated glycolipids of PNS. Immunodetection of the antibodies was performed on thin-layer plates after separation of the glycosphingolipids from peripheral nerve and white matter. The lipid antigen had a migration on thin-layer chromatography close to sulfogalactosylceramide but the sulfatide standard did not react with the monoclonal IgM.