The in vitro antibacterial activity of cefozopran (CZOP) against recent clinical isolates was evaluated and compared with those of ceftazidime (CAZ), cefpirome (CPR), cefepime (CFPM), cefotaxime (CTX), sulbactam/cefoperazone (S/C), imipenem (IPM), oxacillin (MPIPC), and flomoxef (FMOX). MIC80 values of CZOP for methicillin-susceptible Staphylococcus aureus (MSSA, n = 41), methicillin-resistant Staphylococcus aureus (MRSA, n = 57), Streptococcus pneumoniae (n = 45), Enterococus faecalis (n = 49), Enterobacter cloacae (n = 50), Citrobacter freundii (n = 45), Serratia marcescens (n = 45), and Pseudomonas aeruginosa (n = 100) were 1, 32, 2, 16, 4, 1, 0.25, 8 micrograms/ml, respectively. CZOP was more active than CPR against P. aeruginosa and exhibited similar activity to CPR against other species. CZOP was especially active against S. marcescens with MIC values lower than 1 microgram/ml against all strains tested. CZOP was similarly active to or more active than CFPM against all species except for C. freundii. CZOP was not active against MRSA. Thus, we investigated the in vitro combination effects of CZOP/vancomycin (VCM) and CZOP/arbekacin (ABK) using the checkerboard method. The interaction between CZOP and VCM ranged from additive activity (0.5 < FIC index < or = 1.00, n = 37) to synergistic activity (FIC index < 0.50, n = 1), except for one strain showing indifference (1.00 < FIC index < or = 2.00). The interaction between CZOP and ABK ranged from additive activity (n = 22) to synergistic activity (n = 1). These date suggest the potential effect of combination therapy of (CZOP) and VCM or ABK against MRSA. The combined therapy is suggested to be useful to reduce side effects in patients with impared renal function, to reduce the administration dose of VCM or to treat infections of sites where achievable drug concentrations are lower than those commonly achieved in the bloodstream. We also investigated the combination effects of CZOP/AMK and CZOP/GM against CZOP-resistant P. aeruginosa (MIC > 16 micrograms/ml). The combination of CZOP/AMK showed additive activity (n = 9) to synergistic activity (n = 2). The combination of CZOP/GM showed additive activity (n = 5). These results suggest that combinations of CZOP with AMK or GM are effective in treating P. aeruginosa.