We investigated the in vitro and in vivo combination effects between panipenem (PAPM) and vancomycin (VCM) on highly methicillin-resistant strains of Staphylococcus aureus (MRSA) isolated from various clinical specimens. Examination of combination between panipenem and vancomycin using checkerboard titration showed a good effect with mean fractional inhibitory index of 0.32 +/- 0.12 on 40 MRSA strains, and the effects were judged as synergistic against 33 strains (83%) and additive against 7 strains (17%). In the combination of PAPM and VCM at 1/4 MIC each against exponentially growing MRSA, bactericidal activity was found when PAPM was added at 1 hour or 2 hours prior to VCM-addition, and PAPM with VCM was added simultaneously, although bactericidal activity was scarcely demonstrated when VCM was added at 1 hour or 2 hours prior to PAPM-addition. Bactericidal activity was enhanced against MRSA in the combination of PAPM and VCM at 1/4 MIC each for MRSA than the bactericidal activity of VCM at 1 MIC alone, and the combination showed a strong bactericidal activity against P. aeruginosa. VCM alone, however, had no bactericidal activity in the in vitro mixed cultures of the two bacteria. Furthermore, the combination of PAPM and VCM induced a marked damage to cell surface and bacteriolysis against MRSA and P. aeruginosa in the mixed cultures, although VCM alone induced only slight morphological alterations. Penicillin-binding proteins (PBPs) including MRSA-specific PBP 2' were decreased greatly in the amounts in MRSA-cells with the increase of VCM-treated concentration. The combination therapy of PAPM and VCM showed a greater efficacy than the therapeutic efficacy of each antibiotic alone against mixed infection in burned mice caused by MRSA and P. aeruginosa, and the activity was judged as synergistic based on the FED index smaller than 0.34.