A unique combined luminescence assay for firefly (Photinus pyralis) luciferase and beta-galactosidase (beta-gal) reporter gene products is described. Luciferase and beta-gal activities are determined with the same aliquot of cell lysate prepared from cells contransfected with both reporter genes, thereby reducing manual labor and increasing experimental accuracy. With the Dual-Light assay system, luciferase activity is measured first with an enhanced luciferase assay, followed by quantitation of beta-gal with Galacton-Plus chemiluminescent substrate and Sapphire-II enhancer. Highly sensitive detection of luciferase (2 fg) and beta-gal (8 fg) is achieved with a dynamic range over seven orders of magnitude of enzyme concentration. Comparative analysis of both independent and combined (Dual-Light) detection methods for cells contransfected with luciferase and beta-gal reporter genes is also described.