Discrepant results are obtained when antibiotic-induced bacterial killing is quantitated by viable counting or bioluminescence assay of intracellular ATP. In this study the killing of bacteria exposed to amikacin or imipenem was quantitated by viable counting and bioluminescence assay of intracellular ATP and also by quantal analysis. The results of quantal analysis and viable counting agreed very well, whereas a much lower degree of bacterial killing was recorded by bioluminescence.