METHODS: A mixed culture of a wild-type ceftazidime/cefepime-susceptible (4x107 CFU/mL) strain and an ampC derepressed Enterobacter cloacae (105 CFU/mL) strain (relative proportions 99.75% and 0.25%) was challenged for 4 h with different antibiotic concentrations of ceftazidime and cefepime (0.03-4096 mg/L), and then transferred to drug-free medium. The proportion of wild-type versus derepressed population was evaluated after 24 h. RESULTS: Ceftazidime and cefepime selected the derepressed variant at concentrations ranging from 1 to 4096 and from 0.12 to 16 mg/L respectively. CONCLUSIONS: These results suggest that serum concentrations attainable with a 2 g/8 h cefepime dosage may be able to suppress the emergence of derepressed ampC mutants.