Borrelia hermsii and other relapsing fever (RF) species are noted for their highly polymorphic surface antigens, the variable major proteins (VMP). Less is known about other surface proteins of these pathogens in either their vertebrate reservoirs or arthropod vectors. To further characterize these proteins, we elicited antibodies against VMP-less cells, noted antibody reactions against whole cells and cell components, and then subjected selected antigens to mass spectroscopy for amino acid sequencing for comparison against a B. hermsii genome database. One of the derived monoclonal antibodies, H0120, agglutinated spirochetes, and in Western blot analyses, it bound to a 14-kDa protein of whole cells and their membrane fractions but not after protease treatment. A search of open reading frames of the B. hermsii genome with extracted peptides identified the 14-kDa protein with bha128, a 453-nucleotide gene of the 175-kb linear plasmid. The bha128 gene was synthesized and expressed in Escherichia coli. The protein product was bound by antibody H0120. Genes homologous to bha128 occur in the RF species Borrelia turicatae, B. duttonii, and B. recurrentis but not in Lyme disease Borrelia species or other organisms. The following findings indicated an association of BHA128, renamed Alp, with the tick environment: (i) Alp was produced at higher levels at 23 degrees C than at 34 degrees C; (ii) almost all spirochetes in tick salivary glands were bound by the H0120 antibody, but only 1 of spirochetes in the blood of infected mice were bound; and (iii) infected mice produced antibodies to several B. hermsii antigens but not detectably to native or recombinant Alp.