The species-dependent action of type I IFN was analysed by incubating human, canine, murine, and chicken cells for 20 hours with 5 units of type I IFN from either the homologous species or a different one. When cells were treated with the respective homologous IFN an antiviral state was induced as indicated by the lack of GFP expression 6 hours post infection with VSV*DG(Luc) (Fig. 4a) . In contrast, untreated cells or cells that had received type I IFN from a different species were not protected and showed GFP expression accordingly. To compare the effects of different concentrations of homologous and heterologous type I IFN on human cells, we treated NHDF for 20 hours with serial dilutions of human, porcine, and murine type I IFN prior to infection with VSV*DG(Luc). Firefly luciferase reporter activity in infected cell lysates indicated that porcine IFNa is active in NHDF albeit at reduced levels compared to human IFN-b (Fig. 4b; p#0 .0049 for the 0.008 to 2.0 units range). Murine IFN-b showed an even lower activity on NHDF (ED 50 of 500 units per 5610 4 NHDF cells; p#0.0003 for the 0.008 to 500 units range). These results demonstrate that type I IFNs act in a speciesdependent manner and accentuate the need for selecting appropriate cell lines to determine their activity.