Biological response modifiers are increasingly used in viral and cancer therapy. Since alterations of the immune system are the primary symptoms of HIV infection, especially therapies directed towards the modulation of the immune response have been under intense evaluation. This volume summarizes current knowledge of the interferon-based natural antiviral protection system including 2'',5''-oligoadenylate and double-stranded RNA. It will also help to develop further a solid scientific rationale for the practical use of heterologous immunomodulators in the clinics.