Interferon was first discovered in 1957. Over the last five years it has become almost a household word. Many believe it to be a drug with already proven efficacy against cancer and viral infection. The media has distorted any cool scientific view of the data available. We have learned much about the complexity of the interferon system. We know some of the switches involved in interferon gene expression and its secretion by virally and immune stimulated cells. We also know that it binds to a cell surface receptor, mediating its complex effects on target cells by a series of second messengers. The advent of the new techniques of modern molecular biology, such as monoclonal antibodies and gene cloning, has had tremendous impact on the rate of acquisition of knowledge. Such techniques have provided us with almost unlimited quantities of highly purified interferon for clinical trial in patients with a spectrum of infectious and malignant diseases. The information we have gathered raises many more questions. Why should there be several families of interferon genes? What is their true physiolo­ gical role? How are they interrelated functionally? Interferon is clearly a cellular hormone providing a means of communication between cells. ~fhether it has clinical value in the management of patients with diseased cells remains to be seen. This book sum­ marises our current knowledge of interferons as possible an- cancer agents. It is clear interferon is no penicillin for cancer.