The antigenic relationship between T. denticola, T. socranskii subsp. Socranskii, T. socranskii subsp. Buccale, T. socranskii subsp. paredis, Treponeme D, T. vincentii, T. phagedenis biotype Reiter, T. minutum, and T. refringens was studied using fluorescent antibody staining and those organisms were found to contain a group of common antigens. The cross-reactivity was removed when the antisera was absorbed with Reiter cells. There was a second group of common antigens that were shared by the subspecies of T. socranskii, Treponeme D, and T. pectinvorum. Triton-extracts from different treponemal species were found to contain very few cross-reacting antigens as demonstrated by immunodiffusion, crossed immunoelectrophoresis, ELISA, and Western blots. An immunodiffusion test using treponemal Triton-extracts and antisera against five treponeme species was developed for the routine identification of oral treponemes that were in either pure or mixed cultures. T. denticola cells were found to bind to fibronectin-coated microscope slides. Proteins, that bind fibronectin, in the Triton-extracts of T. denticola, were detected in a fibroectin-captured ELISA. The Triton-extracts also reacted with rabbit fibronectin antiserum and rabbit albumin antiserum in immunodiffusion and ELISA assays. Three proteins with molecular weight of 60K, 47K, and 21K, were eluted from a agarose-gelatin-fibronectin column and reacted in Wester blots with T. denticola antiserum. / M.S.