Campylobacter jejuni was isolated from stool specimens of 3 patients with Miller Fisher syndrome (MFS) and 2 patients with Guillain-Barré syndrome (GBS). Anti-GQ1b antibodies in serum from all MFS patients cross-reacted with sialidase-sensitive epitopes in the lipopolysaccharide fraction of C. jejuni from these 3 MFS patients. One GBS patient had anti-GM1 antibodies that bound with lipopolysaccharide of C. jejuni from a control patient and from the other GBS patient without anti-GM1 antibodies. This binding was inhibited by cholera toxin but not by pretreatment with sialidase. The C. jejuni isolate from the GBS patient with serum anti-GM1 antibodies did not contain anti-GM1 antibody-binding epitopes. Our results strongly support the hypothesis that anti-GQ1b antibodies in MFS patients are induced during the antecedent C. jejuni infection. In GBS patients, mechanisms other than molecular mimicry may also be involved in the production of anti-GM1 antibodies.