The mixed haemagglutination technique (MA) was used to demonstrate antibodies to peripheral nerve tissue (PNS) in sera from patients with acute Guillain-Barré syndrome (GBS). Within the first 2 weeks of the disease IgG antibodies were detected in sera from 15 of 42 patients (36%). Antibodies were detected in 60% of the patients with severe GBS, and in 30% of those with a mild GBS. The highest titres were found in the sera from the most severely affected patients and could be detected after 48 days of the illness in one of the sera. Sera from healthy blood donors and from patients with various other diseases did not contain antibodies to PNS. The MA technique is reproducible and is more sensitive than the indirect immunofluorescence test and the antiglobulin consumption test.