The antibacterial activity of BO-2727 against 100 clinical isolates of Streptococcus agalactiae (20 strains), Escherichia coli (20 strains), Peptostreptococcus magnus (20 strains), Bacteroides fragilis (20 strains) and Prevotella bivia (20 strains) was compared with those of imipenem, panipenem, meropenem, biapenem and ceftazidime (CAZ). Minimal inhibitory concentrations (MICs) for 50% of tested strains (MIC50s) of BO-2727 against S. agalactiae, E. coli, P. magnus, B. fragilis and P. bivia were 0.05, 0.05, 0.1, 0.78 and 0.78 mg/l, respectively. MIC90s of BO-2727 against S. agalactiae, E. coli, P. magnus, B. fragilis and P. bivia were 0.05, 0.05, 0.1, 0.78 and 0.78 mg/l, respectively. The efficacy of BO-2727 was evaluated using a polymicrobial pyometra model (E. coli and B. fragilis) in rats. Both the inflammatory changes in the uterus, from the point of view of neutrophil accumulation, and the bacteriological response in the treated group were milder than those in either the nontreated group or the CAZ-treated group. These results indicate that BO-2727 may be useful for the treatment of polymicrobial infections in patients with pyometra.