With the use of a mixed hemagglutination test (MHT), circulating antibodies against human peripheral nerve tissue (PNT) were found in 22 of 38 patients with Guillain-Barré syndrome (GBS) or chronic relapsing demyelinating inflammatory polyneuropathy (CR-DIP). The antibodies were found in only 1 of 34 patients with polyneuropathy of other origin, and in 3 of 32 patients without polyneuropathy. The presence of antibodies was related to response to intravenous gamma globulin treatment in CR-DIP patients. Antibodies against PNT may be pathogenic.