The specificity of the interaction between Treponema pallidum and fibronectin was demonstrated. Treatment of host cells with only antifibronectin sera and not anticollagen or antilaminin sera, inhibited treponemal cytadsorption. Incubation of fibronectin-coated coverslips with monoclonal antibody to the cell-binding domain of fibronectin reduced treponemal attachment to the same extent as antifibronectin serum. Both iodinated fibronectin and iodinated cell-binding domain bound to T. pallidum in a saturable manner. Specificity of the T. pallidum association with the cell-binding domain was the most effective inhibitor of the binding of either radioiodinated cell-binding domain or fibronectin to T. pallidum. Scatchard analysis gave Kd on the order of 10(-7) M for both cell-binding domain and fibronectin binding to T. pallidum, consistent with the high affinity interaction of these organisms with host cell surfaces. Finally, the same level of attachment of treponemes was achieved on coverslips coated with cell-binding domain as that observed for organisms incubated with fibronectin, indicating that the cell-binding domain polypeptide is functionally identical to fibronectin in mediating T. pallidum adherence.