Summary The authors report a case of bacteremia caused by cephalosporinase-producing Serratia marcescens, and successfully strictly treated by concomitant administration of sulbactam (Sulb) with combined cefepime (CF) and amikacin (Akn). The data was collected by determining the speed of bactericidal effect. The patient was a 52-year-old man, presenting colectasia and nosocomial bacteremia caused by cephalosporinase-producing S. marcescens. Combined antibiotic treatment with imipenem (IPM) and Akn was given from D1 to D7. At D7, IPM was replaced by CF (4 g/d). Blood cultures remained positive for the same bacteria from D1 to D8. The dosage of CF was increased to 6 g/d (D9) and the patient's catheters changed (D11), without improvement. The same strain of Serratia was isolated from the central catheter (D11) and from stool cultures (D9 and D14). Sulb was given on D14 without modifying the other medication. From D15, fever dropped (t° < 38°C), and was followed by definitive apyrexia. No further Serratia was isolated after D14. Combined CF-Sulb treatment was maintained until D38, and the patient was discharged on D46. Microbiology: the strain of S. marcescens involved (API 20E panel) is resistant to ticarcillin + clavulanic acid (MIC = 128 mg/l), piperacillin (PIP) (MIC = 64 mg/l), PIP + tazobactam, cefotaxime (MIC = 16 mg/l), and fluoroquinolones. It is susceptible to ceftazidime (CAZ) (MIC = 1 mg/l), cefepime (MIC = 1 mg/l), cefpirome (MIC = 0.25 mg/l), imipenem (MIC = 0.25 mg/l) and amikacin (MIC = 1 mg/l) (MIC determined by dilution test in agar). The speed of bactericidal effect against S. marcescens was determined for Sulb alone, CF alone, and combined CF-Akn and CF-Akn-Sulb. A decrease of 4 log10 units was taken to indicate bactericidal activity. The results were as follows: sulb was inactive at a concentration of 32 mg/l, a fall of 2 log10 units at t = 6 h with renewed growth of bacteria was recorded at CF concentrations of 2 to 16 mg/l, fall of 2 log10 units at t = 6 h followed by bacteriostasis was recorded for the combination CF (4 mg/l)-Sulb (16 mg/l), bactericidal effects of CF (4 mg/l)-Akn (2 mg/l) after 5 h 30 min., bactericidal effects of CF (4 mg/l)-Akn (2 mg/l)-Sulb after 3 h 30 min. Conclusion: the failure of IPM-Akn and CF-Akn combinations was neither due to inadequate dose nor to deep-lying infection. Inactivation of cefpirome and CF by cephalosporinases was low, and they were consequently active against Enterobacteriaceae that produce high levels of cephalosporinase. Nevertheless, the determination of the speed of bactericidal activity showed that in this case, CF alone exhibited no bactericidal activity, even at a concentration 16 times greater than the MIC, whereas combined use of Akn conferred bactericidal activity, which was enhanced by Sulb. This correlates to the therapeutic outcome.