Polypeptide and Western immunoblot profiles of subcellular fractions of Treponema denticola ATCC 33520 have been determined by SDS-PAGE of Triton X-100-soluble and -insoluble fractions, a lipopolysaccharide-enriched fraction and purified flagella. Major Triton X-100-soluble polypeptides of 72, 68, 54 and 52 kDa were detected. The 54 kDa polypeptide appeared to be a breakdown product of a larger, heat-modifiable polypeptide. Based on the results of SDS-PAGE analysis and immunoblotting of proteinase K digests of T. denticola, a 'rough' lipopolysaccharide appeared to be present. Electron microscopy has been used to monitor the effect of detergent treatment on the morphology of the organism and to examine the detailed structure of the flagella. Treatment with Triton removed the T. denticola outer membrane, resulting in exposure of the flagella. The flagella were shown to have a complex sheath and core structure and polypeptide composition characteristic of that observed for other treponemes. Polypeptides of 38, 35, 32 and 28 kDa were present in purified flagella preparations. Immunoelectron microscopy, iodine-labelling and Western blotting were used to demonstrate the exposure of antigens on the T. denticola surface. Surface iodination located polypeptides of 72, 68 and 54 kDa. Antiserum raised against whole cells of T. denticola recognized these polypeptides and an additional polypeptide of 52 kDa. These data provide a basis for future detailed molecular analysis of the ultrastructure and antigenicity of T. denticola.