Human granulocytic ehrlichiosis (HGE) is a rapidly emerging tick-borne infection which presents as an acute febrile illness and is associated with hematologic abnormalities, elevated hepatic transaminase levels, and characteristic intracellular organisms in peripheral blood granulocytes. Although HGE has been successfully treated with tetracyclines, its susceptibility to other antibiotics remains unknown. No clear treatment alternative exist for young children, pregnant women, or allergic individuals, in whom tetracyclines are contra-indicated. We performed in vitro antibiotic susceptibility tests with this recently isolated agent grown in the human promyelocytic leukemia cell line HL-60. Doxycycline (MIC, 0.25 micrograms/ml), rifampin (MIC, 0.5 micrograms/ml), rifabutin (MIC, < or = 0.125 micrograms/ml), ciprofloxacin and ofloxacin (both with MICs of 2 micrograms/ml), and trovafloxacin (MIC, < or = 0.125 micrograms/ml) ciprofloxacin and ofloxacin (both with MICs of 2 micrograms/ml), and trovafloxacin (MIC, < or = 0.125 micrograms/ml) demonstrated significant activity against the HGE agent. These agents were also bactericidal. The HGE agent was resistant to clindamycin, trimethoprim-sulfamethoxazole, and imipenem-cilastatin, as well as to ampicillin, ceftriaxone, erythromycin, and azithromycin, antibiotics commonly used to treat Lyme disease. Both chloramphenicol and gentamicin had weak inhibitory activities but were not bactericidal. Our findings confirm the observed clinical efficacy of doxycycline and further suggest that the rifamycins and quinolones, particularly trovafloxacin, hold promise as alternative agents for treating this new infection.