The effect of interferon in animals infected with some oncogenic viruses can be attributed to inhibition of viral replication or inhibition of the early events following viral infection. In those experimental systems in which continued administration of interferon proved effective after inoculation of oncogenic viruses, the effect may be due in part to an antiviral effect and in part to a direct action of interferon on the multiplication of precursor cells or tumor cells themselves. In animals bearing autochthonous tumors or grafted with transplantable tumors, the effects appear to be due in part to a a direct inhibition of the multiplication of the tumor cell (or an effect on the behavior of these tumor cells) and in part to an effect on the host - the nature of which remains to be defined. We must learn more of the interaction of interferon with host cells and understand how so many apparently different effects are triggered by this "polypractic substance" (6) before we can clarify the mechanism by which interferon can confer protection on animals infested with viruses, protozoa, or tumor cells.