Viral interference is a phenomenon for which a cell infected by a virus becomes resistant toward a second outcoming infection by a superinfectant virus. Even though other mechanisms are known, it can be assumed that most cases of viral interference occurring in natural conditions are mediated by interferon, a low molecular weight protein produced by the infected cell in response to a stimulus provided by viral nucleic acid(s). The interferon produced by a cell can migrate to other cells not yet involved by the spreading infection, transmitting to them the antiviral-resistant state. Available evidence indicates that interferon acts by inducing the production of a second cellular protein, called antiviral protein, which is directly responsible for the antiviral state through some alterations of the cellular, virus-directed, proteosynthetic system. In addition to the antiviral activity, the interferon system can affect the growth of several nonviral organisms and that of tumour cells; rather controversial effects have been shown also on the immune responses; the mechanisms underlying these effects are still nuclear. However a relationship to the specific immune system is suggested also by the finding that interferon can be liberated by sensitized T-lymphocytes following antigenic stimulus. Activation of the interferon system can be operated in vitro and in vivo also by several non-viral substances of various nature, such as nucleic acids, polysaccharides, aromatic amines, etc. This fact, considering that interferon has been shown to play a critical role on the mechanisms of recovery from viral infections, may open new perspectives for their possible prophylactic and/or therapeutic use in viral diseases. This problem can be approached also by administering exogenous interferon. Encouraging preliminary results have so far been obtained either with interferon or its inducers. However, several problems of various nature have to be resolved before considering the actual use of interferon system as a wide range antiviral drug in natural viral diseases of man.