The postantibiotic effect (PAE), the postantibiotic sub-MIC effect (PA SME) and the sub-MIC effect (SME) of a new carbapenem, meropenem, were determined for different strains of Escherichia coli and Pseudomonas aeruginosa. A PAE was induced by 10 x MIC of meropenem for 2 h. After induction, the antibiotic was eliminated by washing and dilution. The strains in the postantibiotic exposure phase and the controls were then exposed to no antibiotic or to different sub-MICs concentrations (0.1-0.6 x MIC) of meropenem. The growth curves were followed for 24 h with viable counts and/or measured by continuous monitoring of optical density in a BioScreen C apparatus. Zero or very low values for the PAE were seen in the strains studied. However, a long PA SME and SME were seen for P. aeruginosa when determined by both methods, which at 0.3 x MIC gave comparable results. At lower concentrations the PA SMEs measured with BioScreen C were shorter, probably due to differences in the definition between the two methods. The BioScreen C method was less laborious than viable counting. However, one disadvantage of the BioScreen C method was that manual viable counting still had to be used to determine bacterial killing. No PA SME or SME against E. coli was observed, and the increased sensitivity of P. aeruginosa to sub-MICs of carbapenems remains unexplained.