Mice experimentally infected with extended-spectrum beta-lactamase-producing Klebsiella pneumoniae strains were injected twice daily for three days with ceftazidime, cefepime, or imipenem (25, 50, or 100 mg/kg/injection). Treatment efficacy was based on five-day survival and on the spleen viable bacteria count 16 hours after the last treatment dose. Under these experimental conditions, ceftazidime showed some activity on strains with low levels of resistance to ceftazidime. Cefepime used in a dose of 50 or 100 mg per injection demonstrated good activity but was slightly less effective than imipenem.