A 65 kDa protein (Bb65) has been identified as one of the major specific antigens of Bartonella bacilliformis, the causative agent of bartonellosis which is a bacterial infectious disease of inhabitants of the Andes. The gene encoding this antigen (7B2) was isolated from an expression library made directly from randomly generated fragments of B. bacilliformis genomic DNA using Bartonella antibodies raised in rabbits and sera of bartonellosis patients. The Bartonella 7B2 gene was expressed in Escherichia coli and the recombinant Bb65 protein was purified by column chromatography. Using polyclonal antibodies raised in rabbits, the antigen was shown to be present in all of 13 B. bacilliformis isolates from different Peruvian regions. Immune electron microscopy demonstrated the probable cytoplasmatic localization of Bb65. When applied to enzyme immunoassays, Bb65 sensitively and specifically bound to IgG antibody of sera of bartonellosis patients, convalescents, and immunes from various Peruvian regions. IgM antibody was not recognized by Bb65, neither was IgG antibody circulating during the first 2 weeks of illness. The amino-terminal amino acid sequence of Bb65 was 53% homologous to the 65 kDa heat shock protein of Mycobacterium tuberculosis.