The engagement of a common cellular receptor by type I IFNs leads to activation of the JAK/STAT pathway and transcriptional induction of several genes with antiviral activity [1, 2] . It is common practice that cells are incubated with type I IFN for several hours to induce an antiviral state [15, 19] . Our findings suggest that an antiviral state can be detected much earlier. For example, treatment of NHDF with 1 unit of human IFN-b for 1 hour and subsequent infection with VSV*DG(Luc) for 5 hours was sufficient to suppress firefly reporter expression by 50%, even though the effective dose was further lowered with longer incubation times. While DF-1 fibroblasts responded to chicken IFN-a with dose response kinetics similar to the one observed with human IFN-b on NHDF, PK-15 cells responded to porcine type I IFN in a much slower way. Thus, compared to conventional bioassays VSV*DG(Luc) may allow quantification of type I IFN in a considerably shorter time provided an appropriate cell line has been selected. The selection of a suitable cell line is also important with respect to the species it is derived from, as full activity of type I IFN was only observed with cells from the corresponding species. Since VSV is able to infect a broad spectrum of mammalian and avian cells, the new bioassay may be used to determine the bioactivity of type I IFNs from many different animals.