Résumé : On Escherichia coli and Pseudomonas aeruginosa the rate of bactericidal activity of netilmicin was high and related to concentrations. The opposite was observed with a beta-lactam antibiotic, piperacillin. For netilmicin, a decrease of 3 log10 was observed within 60 minutes at a concentration of 4 μg/ml. From the bacteriological point of view, high netilmicin concentrations over a short peirod of time are to be preferred rather than a concentration just above the minimum inhibitory concentration over a long period of time, as is the case with piperacillin.