Interferons are cytokines and thus fulfil a vital role in communication between cells in their microenvironment. Type I interferons, the group to which interferon beta (IFN-beta) belongs, share several structural and functional properties by which they distinguish themselves from type II interferon or IFN-gamma. In particular, IFN-beta can be produced by many different cells while IFN-gamma is an exclusively lymphocytic cytokine, i.e. a lymphokine. IFN-beta is functionally linked to other cytokines as it can induced by some of them (e.g. interleukin I) and as its actions can be potentiated or antagonized by other cytokines. Such interactions can take place at several levels, e.g. at the level of signal transduction and transcription activation. Of potential interest for the role of IFN-beta in multiple sclerosis is its ability to function as a deactivator of mononuclear phagocytes, and hence as an inhibitor of inflammation.