The bactericidal effect (BE) of an antibiotic reduces the infective population, and its postantibiotic effect (PAE) assures a persistent inhibition of bacterial cells after a short exposure to the antimicrobial agent. Both effects prevent the early regrowth of the infecting organisms when the antibiotic tissue levels decrease to below the MIC value. The BE and the PAE of imipenem, cefotaxime, ceftazidime, piperacillin, gentamicin and ampicillin on Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Pseudomonas aeruginosa, Acinetobacter calcoaceticus, Staphylococcus aureus and Streptococcus faecalis were investigated with viable counts and continuous impedance monitoring of broth cultures. Imipenem and gentamicin gave similar high BE and PAE values at low concentrations and with short drug exposures in most strains tested. PAE is low or non-existent for Gram-negative strains with other beta-lactam antibiotics. These results suggest the possibility of future clinical studies with new experimental dosage schedules for imipenem.