Expression of reporter genes incorporated into complex viral genomes is being used increasingly to monitor virus infection in cell culture and in the organism. One of the most sensitive markers is luciferase from Photinus pyralis which catalyzes a luminescent reaction that can be traced in cell and organ extracts in luminometers. A novel method is described for monitoring Iuciferase activity after infection of cells with recombinant herpesvirus (pseudorabies virus) which carries stably and expresses luciferase using ultra high sensitive photon-counting enhanced video image acquisition. The data show that firefly luciferase activity in virus-infected cells can be monitored without destruction of the cells using video equipment for either macroscopic image acquisition or microscopy. Resolution down to a single-cell level can thereby be achieved. This method increases greatly the potential of monitoring virus infection in real time with a non-destructive highly sensitive method in cell culture and should also help to assay viral spread in the animal.