Synthetic glucocorticoids belong to the most frequently administered drugs in livestock production. These synthetic hormones are employed for therapeutic purposes against inflammatory reactions, disorders of the musculoskeletal system, bovine ketosis and many other diseases of farm animals. A widespread illegal use of synthetic glucocorticoids to improve feed intake and weight gain has also been observed. To enforce the residue limits imposed on glucocorticoid drugs and preclude their illicit administration as growth promoters, it is necessary to establish high throughput analytical methods that can be applied to the screening of animal tissues. Here, we developed a dual luciferase reporter assay that detects residues or contaminants with glucocorticoid activity. This screening assay is performed by transfection of human cell lines with two reporter constructs followed by the measurement of two distinct luminescence signals, one of which serves as internal control to correct for assay variabilities and unspecific matrix effects. The limit of detection (1.25 microg for dexamethasone in liver) depends on the biological potency of each synthetic glucocorticoid but, with all drugs tested, the maximal response reaches a 20 to 30 fold induction of luciferase activity. In combination with an appropriate sample clean-up method (recovery of 82%), this luciferase assay has been applied to the analysis of liver samples from calves treated with a single therapeutic injection of either dexamethasone or flumethasone. Thus, the dual luciferase reporter assay provides a new screening tool to detect unwanted glucocorticoid activities in animal tissues or other crude biological samples without knowledge of the precise chemical entity of the parent compounds or their metabolites.