Résumé : In order to define whether the mode of action of cefsulodin is the same on two different Gram-negative bacilli, a growth curve study (MS-2 Abbott) was carried out on seven different biotypes of Escherichia coli and seven different lysotypes of Pseudomonas aeruginosa. In agreement with the high MIC of cefsulodin for the E. coli strain these data show a rapid drop in optical density (lysis checked by phase contrast microscopy) in E. coli culture with high cefsulodin concentrations (from 64 to 500 μg/ml). In P. aeruginosa culture a residual growth was observed at all concentrations from 2 to 500 μg/ml. This residual growth was due to long filaments which were not observed in E. coli cultures. This early response of the growth curve was concentration independent. When β-lactamase was added, the lag of regrowth was concentration dependent for E. coli culture, but concentration independent for P. aeruginosa. This observation reflects the different mode of action of cefsulodin (penicillin binding proteins and/or permeability barrier) related to bacterial species.