Laboratory diagnosis of Borrelia burgdorferi is routinely made by an enzyme-linked immunosorbent assay, with positive results confirmed by Western blot analysis. Concern has been raised that false-positive diagnoses may be made on the basis of serologic cross-reactivity with antibodies directed against other bacterial pathogens, in particular the agent of human granulocytic ehrlichiosis (HGE). The present study made use of a mouse model to ascertain the validity of these concerns. Two different strains of mice were inoculated with the HGE agent and assayed for production of polyclonal and monoclonal antibodies to antigens of both of these bacteria. Infection of mice with the HGE agent does not induce diagnostically significant B. burgdorferi serologic cross-reactions.