Axial filaments (flagella) of Treponema pallidum have been purified in large enough quantities to be analysed electrophoretically. They produced a characteristic linear precipitate in two-dimensional immunoelectrophoresis. Polyacrylamide gel electrophoresis showed three major polypeptides, the most prominent having an apparent molecular weight of 37,000, about 1500 less than the corresponding component of axial filaments of the Reiter treponeme. A doublet of less abundant polypeptides of 33,500-34,000 MW also differed slightly from those of the Reiter treponeme. Western blot analysis showed that the principal polypeptide of the T. pallidum axial filament was strongly antigenic, and antibody to it was prominent in sera from hyperimmunized, as well as acutely infected (orchitic), rabbits, and in soluble fractions from acutely infected rabbit testes from which large numbers of viable treponemes had been extracted. This indicated that antibody to this component was ineffective in eliminating treponemes from the tissue.