Antisera prepared against heated cells of 10 strains of Clostridium tyrobutyricum gave cross-reactions with several other clostridial species. Such reactions could be completely eliminated by absorption with C. beijerinckii, C. fallax and C. tetanomorphum. Both agglutination and immunofluorescence tests with these antisera showed that 85 strains of C. tyrobutyricum were divided into two main serological groups, A and B. The 56 strains of group A possessed the same thermostable species-specific antigen, whereas those of group B were lacking in it. Antisera prepared against either formol- or ethanol-treated cells or low-heated cells of C. tyrobutyricum showed only low titre cross-reactions with other clostridial species. When these were removed by absorption, only the homologous strains and a few other strains of C. tyrobutyricum group B were still agglutinated at a full titre by these antisera. Therefore, only strains of group A of C. tyrobutyricum can be easily identified by a single type-specific antiserum.