Using Penner's method and Lior's scheme, we serotyped 31 isolates from patients with Guillain-Barré syndrome, 7 isolates from those with Fisher's syndrome, and isolates from patients with sporadic enteritis. PEN 19 of Campylobacter jejuni was isolated more frequently from the Guillain-Barré syndrome patients (16/31 isolates, 52%) than from the sporadic enteritis patients (5%). LIO 7 of C. jejuni also was isolated more frequently from the Gillain-Barré syndrome patients (14/31 isolates, 45%) than from the enteritis patients (3%). One reason why Guillain-Barré syndrome is rare, despite the high incidence of C. jejuni enteritis, may be the low frequencies of PEN 19 and LIO 7. The frequency of positive anti-GM1 antibody titers in the Guillain-Barré syndrome patients with PEN 19 isolates was higher than that in the Guillain-Barré syndrome and Fisher's syndrome patients without PEN 19 isolates. We speculate that the serotypic determinant of PEN 19 aids in the production of anti-GM1 antibody by a GM1-like lipopolysaccharide. In contrast, 5 of the 7 isolates from the Fisher's syndrome patients belonged to PEN 2:LIO 4. The IgG anti-GQ1b antibody was associated with PEN 2 and LIO 4. These serotypic determinants may aid in the production of IgG anti-GQ1b antibody by a GQ1b-like lipopolysaccharide.