To study the sensitivity of type I and type III IFNs to conditions of extreme pH, human IFN-b and human IFN-l were treated for 30 min with either 0.1 M HCl, 0.1 M NaOH or H 2 O, adjusted to neutral pH, and assayed on NHDF and Calu-3 cells, respectively. It turned out that the antiviral activity of human IFN-b was not significantly affected by acid (p.0.05) (Fig. 5b) , confirming the previously noted acid-stability of type I IFNs [31] . IFN-l showed similar properties as antiviral activity was maintained following treatment with 0.1 M HCl (Fig. 5c) . In contrast, alkaline treatment significantly reduced the activity of IFN-l (p,0.05 for the 250 2 1 ng/ml range), whereas IFN-b was not affected (p.0.05) (Fig. 5b) . Thus, treatment with acid may be employed to inactivate virus in a test sample without affecting the activity of type I and type III IFNs. On the other hand, alkaline may be used to differentiate between type I and type III IFNs.