VSV*DG(Luc) was also used to quantify the activities of porcine and chicken IFN-a. While the dose response curve of chicken IFNa on DF-1 chicken fibroblasts (Fig. 2b) was similar to the one of human IFN-b on NHDF (Fig. 2a) , porcine IFN-a showed a different kinetics on PK-15 porcine kidney cells. In these cells, a full antiviral state was accomplished only after 20 hours of treatment indicating that PK-15 cells respond rather slowly to the action of type I IFN (Fig. 2c) . Thus, the assay is applicable to type I IFN from different species but may perform differentially on distinct cell types. To further evaluate the bioassay, samples containing type I IFN of unknown activity were tested against a commercial IFN-b standard and the results were compared with those obtained with a conventional IFN bioassay based on VSV cytotoxicity. Human type I IFN was induced in NHDF human fibroblasts following infection with VSV*M q [30] . This propagation-competent virus expresses a mutant matrix protein that is unable to block the nucleocytoplasmic RNA transport of the cell. Before the samples were tested for antiviral activity, VSV*M q was inactivated for 30 min with 0.1 M HCl to avoid any interference with the bioassay (see also the section on thermal and pH stability of IFN). When the VSV*DG(Luc) replicon bioassay was used, about 0.8 pg/ml of a commercial IFN-b standard resulted in 50% antiviral activity. In consideration of the final dilution producing 50% antiviral activity, the type I IFN concentration of two test samples was defined as 4000 pg/ml and 1440 pg/ml, respectively (Fig. 3a) . When the samples were tested with the conventional bioassay (Fig. 3b) , the values were in the same range (5000 and 1500 pg/ml), indicating that both assays principally agree. Nevertheless, the replicon-based bioassay proved to be more sensitive than the conventional one as lower amounts of type I IFN were sufficient to reduce the luciferase reporter activity by 50% (compare curves shown in Fig. 3a with those in Fig. 3b ).