The major goal of this study was to achieve sustained cultivation of virulent T. pallidum in vitro. The putatuive binding of host proteins to the outer membrane (OM) of intact, virulent T. pallidum subsp. pallidum has been investigated. A major breakthrough was the development of a filtration assay, usinglow protein-binding membrane filters, for the measurement of substances bound to or incorporated into th eOM of T. pallidum. This avoided the conventional manipulations which can damage the fragile OM of T. pallidum. Using this filtration assay, studies on the binding of host serum proteins demonstrated that intact treponemes did not bind host proteins as previously reported. It also indicated that previous studies were probably performed with damaged by this research. The studies on the binding of polyclonal and monoclonal antibodies to intact and detergent treated treponemes provided evidence of the low level binding of antibody to intact treponemes which was greatly enhanced but the removal of the outer membrane with 0.1% Triton X. This research research corroborated that of others which suggests that the outer membrane of T. pallidum contains very little protein or surface exposed antigen.