ATP production, measured by the luciferin-luciferase assay, is an indicator of bacterial metabolic activity. This enzymatic assay yields rapid results (< 5 minutes), permitting multiple measurements and establishment of ATP growth curves in order to study the kinetics of antibiotics in bacterial populations. The measurement of free or extracellular ATP, total ATP (extra and intracellular) and the ratio of free to total ATP are additional means of studying the bacteriostatic or bactericidal activity of antibiotics. An increase in free ATP is an indicator of extracellular movement due to alteration of the cell wall. The ratio free ATP/total ATP × 100 ≥ 50%, indicates bacteriallysis. These assays were used to study the effects of 14 antibiotics on two reference strains of Escherichia coli ATCC 25922 and Staphylococcus aureus 25923.