BACKGROUND: Human brucellosis is characterized byfocal complications, chronic courses, and therapeutic failures. METHODS: 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. RESULTS: 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. CONCLUSION: 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.