Human brucellosis is characterized byfocal complications, chronic courses, and therapeutic failures. In a relapsed case of brucellosis after short-term antibiotic therapy using doxycycline and ciprofloxacin two Brucella strains were isolated, before and after treatment. In vitro susceptibilities of both isolates were determined by E tests including a great variety of antibiotics. In a killing rate experiment the bactericidal activities of doxycycline, streptomycin, rifampin and ciprofloxacin as single agents and in combinations were determined. Lowest MIC values were measured for doxycycline and ciprofloxacin. MICs did not change under therapy. Streptomycin alone exhibited the most effective killing within 6 h, whereas the other single agents did not show bactericidal activity. Doxycycline plus ciprofloxacin was the most active combination in vitro. Routine susceptibility testing of Brucellae is not obligatory as most of the 'traditional' anti-Brucella antibiotics are active in vitro and bactericidal efficacy may differ in vivo. Copyright 2005 S. Karger AG, Basel.