The detection of bacterial contamination of water, pharmaceutical products etc. is of great importance, and is most conveniently performed by the detection of bacterial ATP (Adenosine TriPhosphate) using the luciferin-luciferase bioluminescence system. This system uses unstable and expensive reagents, and emits transient light signals. In this study an FIA (Flow Injection Analysis) system was set up to monitor the light signal produced by the reaction. Using a luminometric detector (a liquid scintillation counter) with the FIA system, the reaction length, sample volume, flow rates, pH etc. were investigated.