Molecular mimicry, resulting from structural similarities between self-determinants on host Ags and an organism's antigenic determinants (epitopes), can incite autoimmune events in certain bacterial and viral diseases. In the course of comprehensively mapping the 47-kDa lipoprotein (Tpp47) of Treponema pallidum subsp. pallidum using an overlapping synthetic peptide strategy, we identified a major immunoreactive epitope (411PGTEYT416) that exhibited considerable motif identity with multiple repeats of analogous linear sequences found in mammalian fibronectins. To further explore the importance of this motif as a probable instigator in the induction of polyspecific cross-reactive Abs, mimetic variants were synthesized for immunologic studies. Mimetics with ala (A) replacements in each amino acid position were used to determine which residues were critical for Ab binding. Animals immunized with two mimetics (PGTEYT or PGSEYT) coupled to tetanus toxoid exhibited: 1) modified responses when challenged with viable T. pallidum; and 2) classical Arthus reactions when challenged intradermally with either motif linked to a different carrier. The cross-reactive nature of the Ab responses to both mimetics was confirmed in a variety of ELISAs using mimetics, fibronectins, and collagens. Inhibition-ELISA studies with both fibronectin and an unrelated mimetic of the RGD motif suggest that intra- and intermolecular epitope spreading occurs following mimetic immunization and involves additional self-epitopes. These observations suggest that although molecular mimicry plays a pivotal role in initially triggering the anti-fibronectin and anti-collagen responses associated with disseminated syphilis, expansion of those autoimmune responses may be due to other self-epitopes once tolerance is abrogated.