Various viruses have been shown to cause colds,and prophylactic vaccination is unlikely to prove practical. An alteranative is the use of specific antiviral compounds for both prophylaxis and therapy. Experiments using human volunteers have shown that purified natural (leukocyte) and cloned (alpha) interferon can prevent colds, but that long-term administration of either in effective amounts causes nasal irritation. A synthetic antirhinovirus drug, enviroxime, has also proved active when given prophylactically. While the results of these experiments are encouraging, there are still considerable difficulties to be overcome. Interferon is clearly to toxic for all but short-term prophylaxis. The activity of synthetic antivirals remains barely detectable in vivo, probably because of their poor bioavailability, which may prove to be a stumbling block to future progress.