After a brief historical introduction, early work on the purification of interferons, which led over 20 years to the isolation of pure human and mouse interferon, is described. The way in which interferons are made is discussed; cellular genes are activated by processes, still only partially understood, to produce interferon messenger RNA which is then translated, glycosylated and secreted to give the final product. Interferons act by protecting the cell rather than inactivating the virus, and the multitude of cellular processes affected by interferons are described—some, but not all, needing the involvement of double-stranded RNA Thus virology, cell biology, genetic engineering and biochemistry have all been used to prepare the large amounts of human interferons now being tested in clinical trials.