In this study we have evaluated the bactericidal activity kinetics of some cephalosporins, ureidopenicillins and aminoglycosides against 41 gram negative clinical isolates. Aminoglycosides, particularly gentamicin, showed the better bactericidal activity which appeared already at the second-third hour after the addition of the antibiotic. The bactericidal activity of cefotaxime, ceftazidime and cefotetan was remarkable but it appeared only at the sixth-eight hour. This difference was statistically significant. Carbenicillin and piperacillin showed a low bactericidal activity against Pseudomonas aeruginosa. We noted a wide variability of the antibiotic bactericidal activity against the isolates of the same bacterial species. Therefore we can argue that in serious infections the determination of the antibiotic and serum bactericidal activity is necessary.