We often encounter the problem that the activity of type I IFN has to be determined in a sample containing live virus. As virus infection may interfere with the bioassay, it has to be inactivated before the assay is performed, preferentially without touching the activity of type I IFN. As many viruses can simply be inactivated by treatment with heat, we first analysed the thermal stability of human IFN-b using the VSV*DG(Luc) bioassay. The antiviral activity of human IFN-b was fully maintained when the cytokine was incubated for 30 min at 50uC (Fig. 5a) . At 60uC, 99% of the activity was preserved (p = 0.021). At 70uC, the activity of human IFN-b dropped to 83% (p = 0.0017). Incubation at higher temperatures (80uC and 90uC) affected the activity more drastically, although some activity was still left at these temperatures. Only when human IFN-b was heated to 100uC for 30 min, activity was completely abolished. In contrast to human IFN-b, porcine IFN-a was completely inactivated at 70uC (p,0.0001), whereas human IFN-l (IL-29), a type III IFN, showed even higher residual activity at 80uC and 90uC (p,0.001), suggesting that these cytokines have different physicochemical properties.