A study was performed to compare viable counts and bioluminescence for determining control related effective regrowth time (CERT) and postantibiotic effect (PAE) on Gram-negative bacteria after two hours of exposure to meropenem. There was a good correlation between bioluminescence and viable counts in determining the cell numbers in growing cultures of Escherichia coli. CERT was defined as the time required for the resumption of logarithmic growth and an increase of 1 log10 to occur over the pre-exposure inoculum in the test culture minus corresponding time for the control culture. PAE and CERT were studied on reference strains of Enterobacter cloacae, E. coli, Klebsiella pneumonias and Pseudomonas aeruginosa. At 4 × MIC of meropenem the CERTs of these four Gram-negative strains were 41, 4.9, 4.2, and 3.6 h, respectively, when assayed by bioluminescence. Corresponding CERTs using viable counts were 4.2, 5.0, 5.1 and 3.8 h, respectively. In contrast to this good agreement between the methods in assessing CERT, the corresponding PAEs were highly method dependent. At 4 × MIC of meropenem the PAEs on E. cloacae, E. coli, K. pneumoniae and P. aeruginosa were 3.9, 4.8, 4.7, and 3.5 h, respectively, when assayed by bioluminescence. However, the corresponding and simultaneously determined viable count PAEs were −0.4, 0.5, −0.1 and 0.7 h, respectively. The poor correlation between these methods in assessing the PAE is caused by greater initial decrease in viability compared with the less prominent initial change in cell density as measured by bioluminescence. Since this initial change in cell density is the start value for the PAE calculation, these discrepancies jeopardize the possibility of comparing PAE results obtained with different methods. However, the use of CERT can circumvent these methodological problems and is suggested for further pharmacodynamic studies.