A novel assay to study transcriptional regulation in vivo designated trans-activation-dependent replication (TDR) assay is based on the modulation of a simian virus 40 (SV40)-derived replication system. A mixture of four plasmids (pPARA + pCIS + pTRANS + pREF) is co-transfected into vertebrate cells. After appropriate incubation, the replication of the pPARA plasmid (containing an SV40 origin of replication) is measured with a simple enzymatic test. We demonstrate that the level of replication is dependent on the differential trans-activation of the reporter pCIS (in which SV40 T-antigen is brought under control of the desired promoter) by the specific regulator protein encoded by the pTRANS plasmid. Three advantages make this assay a convenient tool for the systematic analysis of trans-activation in vivo: (1) remarkable sensitivity (higher than conventional assays); (2) rapid sample processing combined with a built-in standard (pREF-plasmid); (3) avoidance of expensive reagents such as freshly radiolabelled probes. We present the application of the TDR assay to the analysis of deletion mutants of the glucocorticoid receptor (GR) and other, GR-based chimeric trans-activators. The results demonstrate that the properties of protein domains are not always additive in a particular chimaera. Further application possibilities of the TDR assay are also discussed.