The effects of interferon were probably encountered, though not recognized as such, in the 1930’s 1 , 2 through the phenomenon of viral interference. It remained for Isaacs and Lindenmann 3 in 1957 to show that this effect was mediated by a protein which they called interferon. Their discovery was made when they found that chick chorioallantoic membranes which had been exposed to heat-inactivated influenza virus released a protein factor into the incubation medium which was capable of protecting fresh pieces of membrane from virus infection. These observations 124have stimulated a great deal of research into the mechanism by which viruses induce the synthesis of interferon. Since that time members of all the major classes of viruses have been found to have the ability to induce interferon.