P 450-Glo Luminescent P 450 assays utilize derivatives of beetle luciferin as luminogenic probe P 450 substrates. Luciferin is a substrate for the light generating reaction of firefly luciferase. The derivatives are not substrates for luciferase but are metabolized by P450s to luciferin, which in turn reacts with luciferase to make light. Assays are easily configured in multi-well plates where a homogeneous luciferase mixture is added directly to P 450 reactions that use luciferin derivatives as substrates. The amount of luciferin produced by a CYP450 is directly proportional to light output so light is used to measure P 450 activity. P 450-Glo assays were used here to measure CYP1A2, 2C9, 2C19, 2D6 and 3A4 activities and their inhibition by chem. inhibitors. The P 450-Glo assays provided a simple, homogenous and robust luminescent assay format for rapid screening of compounds against P 450 activities.