Homogeneous reagent system that enables fast and simple quantitation of a stable luminescent signal from 2 reporter genes in a single sample. This convenient "add-and-read" system generates both firefly and Renilla luciferase luminescence signals from cells that have not been preconditioned or prelysed. The system is optimized for batch processing both 96- and 384-well plates and is compatible with a wide variety of mammalian cell culture media. The signal half-life is ~ 2 hours for both the firefly and Renilla luminescence.