We have developed a multiplex reporter system to monitor multiple biological variables in real-time. The secreted Gaussia luciferase was fused to ten different epitope tags (Gluc tag), each expressed in different tumor cells. By immunobinding of the tags followed by Gluc tag detection, this system allowed the independent and real-time monitoring of mixed cell cultures in vitro and of mixed subcutaneous and intracranial tumor subpopulations in vivo.