The study described here was carried out to further characterize reference strains of Serpulina (Treponema) hyodysenteriae representing serotypes 8 and 9. Results obtained from restriction fragment length polymorphism analysis, enteropathogenicity testing, and endotoxin profiles confirmed their identifications. Electron microscopy indicated that both strains were covered with a thin layer of capsule-like material. Immunoblot analysis indicated that an antigen in the 19-kDa region of proteinase K-digested whole cells reacted only with homologous antiserum. The serotype-specific antigens were sensitive to periodate oxidation but resistant to proteinase K digestion and migrated in the same region as purified lipopolysaccharides. Immunoblotting with proteinase K-digested whole cells appeared as useful as immunodiffusion with extracted lipopolysaccharide for the serological classification of S. hyodysenteriae. Immunogold labeling of whole cells and purified periplasmic flagella showed strong cross-reactions between S. hyodysenteriae and Serpulina innocens. Outer membrane preparations of strains representing serotypes 8 and 9 contained four major proteins which reacted with antisera against both species, and one major protein with a molecular mass of 46 kDa which reacted only with antisera against S. hyodysenteriae, irrespective of the serotype. Our findings suggest that periplasmic flagella and some outer membrane proteins are antigens common to both S. hyodysenteriae and S. innocens, whereas a 46-kDa outer membrane protein may be a species-specific antigen of S. hyodysenteriae. Finally, we propose immunoblotting as an alternative method to immunodiffusion for the serotyping of S. hyodysenteriae.