Western blot analyses of the human humoral response of patients with Lyme borreliosis have shown that a 17-kDa protein is an immunodominant protein in late disease. Immune electron microscopy with a monoclonal antibody against this protein revealed that the 17-kDa protein is abundantly expressed on the surface of Borrelia afzelii strain PKo. Therefore, the protein has been renamed outer surface protein (Osp) 17. Recombinant Osp 17 of strain PKo was expressed in Escherichia coli and purified by chromatography. Immunoblot analysis of human sera showed a comparable sensitivity with recombinant and natural proteins. The DNA sequences of the osp17 genes from different B. afzelii strains were determined. The DNA sequences of the different osp 17 homologues (six isolates from skin, three isolates from CSF and one isolate from synovial fluid) had high sequence identities of at least 94%. Using a polyclonal antibody against recombinant Osp 17, it was shown that Osp 17 expression varied considerably among the investigated B. afzelii strains. As previously also observed for OspA- and OspC-encoding genes, the osp 17 gene is present in strains not expressing the respective protein. It has been shown that OspA and OspC expression varies in different environments such as tick and vertebrate host. Studies are underway to examine whether this is also true for Osp 17. For diagnostic purposes the use of recombinant Osp 17 has the advantage that the amount of Osp 17 antigen can be easily standardized for immunoblotting, and that this antigen can be used in a protein-specific enzyme-linked immunosorbent assay.