Although type I IFNs are the predominant cytokines with antiviral activity [34] , it cannot be excluded that test samples contain other cytokines that inhibit VSV replication. Indeed, using the VSV*DG(Luc) replicon assay, it was possible to quantify the antiviral activity of IFN-l (Il-29), a type III IFN. This raises the important question of how to distinguish between different antiviral cytokines? Certainly, neutralizing antibodies may be employed to specify the antiviral cytokine present in the sample. In addition, the extraordinary acid-stability of type I IFNs may be used to differentiate them from acid-labile cytokines such as type II IFNs [35] . However, acid may not be used to distinguish between type I and type III IFNs as IFN-l proved to be acid-stable as well. Since IFN-l but not IFN-b was sensitive to 0.1 M NaOH, treatment with alkaline may be used instead. However, the conditions of treatment still have to be optimized to guarantee the complete inactivation of type III IFNs while maintaining the activity of type I IFNs.