Bactericidal action of antibiotics seems to be a test for the choice of antibiotic used and its dosage's adjustment. However, its use is limited by the determination's methods employed until now. By a viable bacteria counting micromethod, bactericidal action of three aminoglycosides: gentamicin, tobramycin and amikacin, is studied towards 48 bacterial strains. For each strain and each antibiotic, 11 antibiotic's concentration have been used, in a dilution range varying from 0.06 to 256 mg x l-1. Viable bacteria counting is executed after 1.5 h, 3 h and 5 hours of incubation. These antibiotics have a dose-dependent action. Regarding the smallest concentration of antibiotic permitted to lower to 3 log10 the bactericidal initial population, one can see that a concentration of 4 mg x l-1 is sufficient for 50% of strains with gentamicin and tobramycin, while a concentration of 16 mg x l-1 is necessary for amikacin. This result is corroborated by a statistical analysis carrying out by a variance analysis: it is showing a little significant difference between gentamicin and tobramycin, and very much significant differences between amikacin and the two others antibiotics.