The success of an invading organism must depend on several cytoplasmic, surface-associated and secreted factors. The technical difficulties in handling pathogenic spirochetes like Treponema pallidum and Borrelia burgdorferi have made it difficult to define specific factors involved in entry and long-term survival. The problem of defining virulence factors has been attacked by several strategies: T. pallidum secretes a number of immunogenic low molecular mass proteins. The most predominant are of molecular weight 15.5 and 22 kDa. Preliminary data suggest that antibodies against these proteins induce protective immunity in rabbits experimentally infected with T. pallidum. Many potentially important surface-associated antigens of T. pallidum have now been cloned and characterized. Two of these, TpD and TpE, are lipoproteins which exhibit characteristic size heterogeneity. The apparent molecular weight of TpE from T. pallidum and T. pertenue are different. The clinical symptoms in syphilis and yaws are very different, but sequence analysis of TpE has shown that the TpE proteins are indeed very similar in the two strains. This observation makes it unlikely that heterogeneity of TpE can account for the different clinical symptoms of syphilis and yaws. Sequence data for another newly sequenced surface-associated antigen of T. pallidum (molecular weight 41 kDa) indicate that this protein is involved in glucose transport and chemotaxis/motility. Intracellular factors like the molecular chaperonin GroEL have been documented both in treponemes and borreliae. This stress protein is involved in cellular repair processes and folding/assembly of protein subunits. Indirect evidence suggests that GroEL affects the ability of spirochetes to survive in the stressful environment of the infected host. Several lines of evidence suggest that the Osp proteins of Borrelia are important for host/parasite interaction. Further support for this idea has come from studies of a series of monoclonal antibodies against OspA. A monoclonal antibody against OspA (9B3D) is able to block attachment of B. burgdorferi to a cell monolayer. Borrelia loses infectivity after several passages in vitro. The loss of pathogenicity is associated with loss of specific plasmids and proteins. One of the low-passage-associated proteins (Lap30) has been cloned and sequenced. Lap30 is a lipoprotein encoded by a 38-kb plasmid, not present in high passage B. burgdorferi. Aberrant immunological processes induced by the lipopolysaccharide component of Treponema hyodysenteriae could explain the dramatic intestinal lesions in swine dysenteriae. But analysis by TLC reveals that the LPS of this treponeme is different from classical Salmonella LPS.(ABSTRACT TRUNCATED AT 400 WORDS)