The in vitro activities of three quinolones (ofloxacin, difloxacin, and ciprofloxacin) were compared with those of trimethoprim-sulfamethoxazole, streptomycin, tetracycline, and rifampin against 47 Brucella melitensis strains. Ofloxacin was the most active of the test antimicrobial agents. It inhibited 90% of B. melitensis strains at a concentration of 0.02 micrograms/ml.