The androgen receptor is an essential regulator of prostate tumour cell growth. We use a cell based mammalian-one-hybrid assay to determine the transcriptional activity of the androgen receptor (AR) based on its interaction with a coactivator. The assay in performed in HeLa cells transfected with the AR expression construct, the coactivator of the AR and a reporter gene consisting of a promoter driving the expression of a firefly luciferase gene. The firefly reporter gene activity is used to determine the activity of the AR. Compounds that inhibit the firefly activity are the desired compounds. The recipient cells are also transfected with a renilla luciferase construct that provides information on the transfection efficiency and at the same time gives an indication of the effect of compounds on general cellular activity (index of toxicity). The ratio of the firefly over the renilla activity gives information on the efficacy of the compounds.