We present the results of sensitivity of 191 Gram-negative bacteria towards the following antibiotics: aminoglycosides (amikacin, gentamicin, tobramycin, netilmicin), cephalosporines (moxalactam, cefotaxime, ceftazidime) and piperacilline obtained by agar diffusion method (Kirby-Bauer) versus automatic system MS 2 Abbott. Essential accord expressed in percentage is for amikacin 96%, for gentamicin 93%, for moxalactam 91%, for tobramycin 95%, for cefotaxime 92%, for piperacilline 89%, for netilmicin 90%, for ceftazidime 92. Full accord gives more discrepant results especially for cephalosporins. For aminoglycosides no significative differences were observed between the two methods. For cephalosporins the incidence of discordance was a little more high. A better sensitivity was obtained by Kirby-Bauer method versus automatic system, which can be considered a therapeutical tool as it furnishes rapidly (4 hours) MIC values, useful to establish antibiotic doses.