The antibacterial activity of PD 131628 was determined by the broth dilution method against 105 clinical isolates of Brucella melitensis. Fifty-two and ninety-nine percent of the isolates were inhibited by 0.06 and 0.12 microgram/ml of this quinolone, respectively. One strain which was resistant to 8 other fluoroquinolones, with a minimum inhibitory concentration (MIC) of > 4.0 micrograms/ml, did not show cross-resistance and had an MIC of 2.0 micrograms/ml. Of the 9 fluoroquinolones tested, PD 131628 was most inhibitory followed by CI-960. All the strains were susceptible to conventional drugs like gentamicin, rifampicin, tetracycline and trimethoprim-sulfamethoxazole, with MICs ranging between 0.12 and 1.0 microgram/ml. PD 131628 did not exhibit in vitro synergy with any of the conventional drugs.