This paper introduces a new method to quantify post antibiotic effect. This method was applied to the study of the effect of three aminoglycosides antibiotics on seven strains of Klebsiella pneumoniae. The antimicrobial was removed by filtration, washing with peptone broth and neutralization by heparin. Growth curves were monitored by automated measurement of optical density with Urimat-ATB (bioMérieux). The inoculum fluctuation bias was eliminated with the help of a mathematical analysis of results. Absence of carry-over was controlled for each bacteria-antibiotic combination. A significant post antibiotic effect was observed with the three antibiotics after 2 hours exposure to a concentration equivalent to 2 x MIC. There was no significant difference between the 3 aminoglycosides tested.