Early experiments showed that administration of mouse interferon preparations inhibited the development of viral-induced or spontaneous viral associated leukemias in mice. Interferon α/β was also shown to inhibit the growth of transplantable tumors of different origins in all strains of mice tested. The finding that interferon α/β inhibited the growth of sublines of tumors selected for resistance to interferon α/β indicated the role of interferon induced host mechanisms in the antitumor effects observed. The different host antitumor mechanisms and especially the interaction of interferon α/β with the immune system have been briefly discussed. Injection of mice with a neutralizing antibody to interferon α/β demonstrated the essential role of endogenous interferon α/β in the defense of the mouse against the development of syngeneic, allogeneic and xenogeneic tumors.