The species Bartonella and Brucella are phylogenetically closely related bacteria, both of which can produce chronic infections in humans that are difficult to cure with antibiotics. MICs of antibiotics for both species correlate poorly with the in vivo efficacy of the antibiotics. In this study we have determined MBCs of several antibiotics for this group of pathogens. Only the aminoglycosides were bactericidal, and this correlates well with the usefulness of these antibiotics for the therapy of human brucellosis and chronic Bartonella spp. infections such as endocarditis. Our data indicate that current clinical experience in treating brucellosis may help to define better the optimum antibiotic therapy for Bartonella-related diseases.