The MAO-Glo Assay provides a homogeneous luminescent method for measuring monoamine oxidase (MAO) activity from recombinant and native sources and for testing the effects of test compounds on MAO activity. The MAO-Glo Assay is performed by incubating the MAO enzyme source with a luminogenic MAO substrate. The substrate of the MAO-Glo Assay is a derivative of beetle luciferin. Upon reaction with MAO, the derivative is converted into luciferin, which in turn reacts with luciferase to produce light. The amount of light produced is directly proportional to the activity of MAO. After the MAO reaction has been performed, the reconstituted Luciferin Detection Reagent is added. The reagent simultaneously stops the MAO reaction and initiates a stable glow-type luminescent signal with a half-life greater than 5 hours. This eliminates the need for strictly timed luminescent detection. The MAO-Glo Assay includes a luminogenic MAO substrate, two MAO Reaction Buffers (one that can be used with either MAO A or MAO B enzyme and one that is designed specifically for MAO B), a lyophilized Luciferin Detection Reagent and the Luciferin Detection Buffer. The user supplies the sample material containing MAO. Protocols are configured for multiwell plate formats but easily can be adapted for single-tube applications.