To investigate the molecular basis for heat-stable antigenic diversity in Campylobacter jejuni, lipopolysaccharides (LPSs) from serotype reference strains and serotyped isolates were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis coupled with silver staining and immunoblotting. By silver staining, only low-Mr components, consisting of one major band and as many as three minor bands ranging in Mr from 4,500 to 5,000, were detected. However, by immunoblotting with homologous antisera, 10 of 34 strains were shown to have a series of high-Mr LPS components characteristic of molecules with O side chains of various lengths. Isolates of the same serotype as the reference strain that had high-Mr LPS molecules were also found to have high-Mr LPS and in one case of cross-reacting strains it was found that the cross-reaction was associated with antibodies against high-Mr LPS. The reactions of LPSs with homologous and heterologous antisera indicated that both high- and low-Mr-type LPSs were strain-specific antigens, but in some cases cross-reactions were noted. Evidently, all C. jejuni strains possess low-Mr LPS that is readily detectable by silver staining, but some serotypes also possess high-Mr LPS components that can be visualized by immunoblotting.