Infection of BHK-21 cells with VSV*DG(Luc) using a multiplicity of infection (m.o.i.) of 4 ffu/cell resulted in increasing firefly luciferase reporter activity with time (Fig. 1c) . In all subsequent experiments, luciferase activity was generally recorded at 5 h post infection as the signal-to-noise ratio was sufficiently high at this time. When BHK-21 cells were infected with different virus dose rates, luciferase reporter activity increased linearly between 0.004 and 40 ffu/cell (Fig. 1d) 