The in vitro combination effects of pazufloxacin (PZFX) with various antibiotics were investigated by the checkerboard dilution method using piperacillin (PIPC), tazobactam/piperacillin (TAZ/PIPC), ceftazidime (CAZ), cefozoprane (CZOP), imipenem/cilastatin (IPM/CS), meropenem (MEPM), panipenem/betamipron (PAPM/BP), amikacin (AMK) and isepamicin (ISP) for clinical isolates of 27 Pseudomonas aeruginosa strains, vancomycin (VCM), teicoplanin (TEIC) and arbekacin (ABK) for clinical isolates of methicillin-resistant 26 Staphylococcus aureus (MRSA) strains, respectively. The following results were obtained. 1. For 27 P. aeruginosa strains, the synergistic effects were observed with the combination of PZFX and CAZ or MEPM (11.1%: 3 strains), and PZFX and CZOP or PAPM/BP (3.7%: 1 strain), respectively. The additive and synergistic effects of PZFX were observed with the combination in all beta-lactams tested in the strains more than 50%. No antagonistic effect was observed. The additive effects were also observed with the combination of PZFX and AMK or ISP in the strains more than 50% of the test strains and no antagonistic effect was observed. 2. For 26 MRSA strains, no antagonistic effect was observed with the combination of all antibiotics tested. The indifference was observed with the combination of PZFX and VCM or ABK in the strains more than 60%, and the additive effects were observed with the combination of TEIC in the strains more than 80%. In conclusion, no antagonistic effect was observed in PZFX with the combination of beta-lactams and anti-MRSA agents, suggesting that the combination therapy of PZFX with these antibiotics would be possible to use for the infections caused by P. aeruginosa and MRSA.