In vitro activities of 13 antibiotics were assessed against 85 Brucella abortus isolates from naturally infected cattle in the Republic of Korea during 1998-2006, using broth microdilution test. Tetracyclines showed the most excellent activity against B. abortus, displaying MIC values of 0.5 ${mu}g/ml$ or below. In particular, minocycline showed the lowest $MIC_{50/90}$ values (0.125/0.125 ${mu}g/ml$) in this study. Among four fluoroquinolones tested, ciprofloxacin ($MIC_{50/90}$, 0.5/1 ${mu}g/ml$) and norfloxacin ($MIC_{50/90}$, 8/8 ${mu}g/ml$) had the most and the least activities, respectively. Gentamicin ($MIC_{50/90}$, 1/1 ${mu}g/ml$) was more effective than streptomycin, erythromycin, rifampin, and chloramphenicol ($MIC_{50/90}$, 2/2 ${mu}g/ml$).