Electron microscopy and SDS-PAGE have been used to analyse the polypeptide and antigenic composition of the sheath and core components of the axial filament of Treponema pallidum. The sheath contains a major 37 kDa polypeptide which was solubilized by a combination of trypsin and urea treatments with concurrent loss of binding of anti-37 kDa monoclonal antibody. These studies also indicated some antigenic heterogeneity within the axial filament population. Trypsin treatment alone removed a number of antigenic determinants from the axial filament but left others intact, suggesting differences in their location within the sheath structure. A second 31.5 kDa polypeptide may also be associated with the sheath. The axial filament core comprises at least two components, an antigenically dominant 33.5 kDa polypeptide and a second of 34 kDa. The structure of the axial filament in T. pallidum and Treponema phagedenis biotype Reiterii was similar, but antigenic cross-reactivity of sheath and core components was incomplete.