The role of lipopolysaccharide (LPS) in the serotyping of Campylobacter jejuni based on heat-stable antigens was examined using SDS-PAGE and a silver stain for carbohydrate. None of the 32 type strains of Camp. jejuni expressed long-chain LPS. Rabbit antibodies, prepared to 10 selected strains of Camp. jejuni, reacted with surface-exposed carbohydrate antigens, which were not LPS. This study suggests that the heat-stable antigens of Camp. jejuni, which form the basis for the established Penner serotyping scheme, are probably capsular and not LPS.