To identify and characterize surface proteins expressed by relapsing fever agent Borrelia hermsii in blood of infected mice, we used a cell-free filtrate of their blood to immunize congenic naïve mice. The resultant antiserum was used in western blots of cell lysates, and gel slices corresponding to reactive bands were subjected to liquid chromatography-tandem mass spectrometry, followed by search of the proteome database with the peptides. One of the immunogens was identified as the BHA007 protein, which is encoded by the 174 kb linear plasmid. BHA007 had sequence features of lipoproteins, was surface-exposed by the criteria of in situ protease susceptibility and agglutination of Vtp- cells by anti-BHA007 antibodies, and was not essential for in vitro growth. BHA007 elicited antibodies during experimental infection of mice but immunization with recombinant protein did not confer protection against needle-delivered infection. Orthologous ORFs to BHA007 were found on large plasmids of other RF species, including the coding sequences for the CihC proteins of B. duttonii and B. recurrentis, but not in Lyme disease Borrelia species. Recombinant BHA007 bound both human and bovine fibronectin with Kd values of 22 and 33 nm, respectively, and to C4 binding protein with less affinity. The distant homology of BHA007 and its orthologs with BBK32 proteins of Lyme disease species, as well as with previously described BBK32-like proteins in relapsing fever species, indicates that BHA007 is a member of a large family of multifunctional proteins in Borrelia species that bind to fibronectin as well as other host proteins.