Effective vaccines against tumor antigens have not yet been produced. However, immunomodulators hold much promise in cancer therapy. Such treatments will probably involve using combinations of various immunomodulators together with activated killer cells. Development of vaccines against tumor-causing viruses seems to be a rational approach to preventing the onset of virus-induced cancers. It seems that efficient vaccines have already been developed for hepatitis B virus; such vaccines have the potential to decrease the incidence of its associated hepatoma. However, successful vaccines against RNA-containing tumor viruses have yet to be developed, although they hold much promise.