This study introduces a method that makes possible the direct monitoring of stimulation fluids for microbial contamination. The procedure is based on the assay of adenosine triphosphate (ATP) with luciferin-luciferase. The accuracy of this method is compared to conventional plate counting and nutrient vial dilution techniques. The effects of stimulation fluid additives such as KCl, biocides, buffers, and gelling agents are assessed, and a procedure for the differentiation of bacterial vs. nonbacterial ATP is examined. Data showing the utility of the assay in monitoring biocide effectiveness and predicting batch mixed gel degradation are presented.