Sera from 30 patients with acute Guillain-Barré syndrome were tested for the presence of antibodies to peripheral nerve tissue using the antiglobulin consumption test. An increased binding of IgG was observed in 15 out of 30 sera and in these sera F(ab')2 fragments prepared from IgG retained the capacity to combine with peripheral nerve tissue, suggesting an antigen-antibody interaction. The specificity for peripheral nerve tissue was demonstrated by absorption experiments. F(ab')2 fragments prepared from IgG of normal sera, however, failed to react with the tissue, suggesting a non-specific binding through the Fc part of the molecule. In sera from patients fully recovered from the Guillain-Barré syndrome and from patients with other neuropathies, no increased binding of IgG was observed.