Patients that have received interferon therapy for tumor diseases to date do not total more than a few hundred. Remissions have been reported in some patients with various types of malignant tumors. This is interesting, as interferon therapy represents a new principle in the treatment of tumor diseases. However, prolonged and complete remissions have been rare, and we are not yet in a position to say whether interferon therapy is superior to other treatment methods in current use for any type of malignant tumor. If this proves to be the case for some such diseases, we would still be faced with the tasks of identifying patients that may benefit from the interferon treatment, and of establishing the best procedure for its application. Research toward these ends would be greatly facilitated by the development of methods for producing interferon.