The bactericidal activity of biapenem, a new carbapenem, against various efflux-mutants of Pseudomonas aeruginosa was compared with those of imipenem, panipenem, meropenem and ceftazidime. The bactericidal activity of biapenem against P. aeruginosa KG5001, a strain deficient in MexAB-OprM, MexCD-OprJ and MexXY-OprM, was very strong compared with those of imipenem and meropenem. In terms of bactericidal activities, biapenem and imipenem had similar activities against P. aeruginosa KG5003, a strain overexpressing MexAB-OprM, as against P. aeruginosa KG5001, however meropenem and ceftazidime had weaker activities against KG5003 than KG5001. The bactericidal activity against P. aeruginosa KG5007, a strain overexpressing MexCD-OprJ, was observed only by biapenem. The bactericidal activity of biapenem was strong and not influenced by all of these three efflux systems.