The genetically determined resistance towards orthomyxoviruses exhibited by mice homozygous (A2G) or heterozygous (A2G X A/J) for the gene Mx was abolished or greatly diminished by treatment with anti-interferon globulin (AIF). AIF induced increased susceptibility to challenge with hepatotropic, neurotropic, and pneumotropic strains of influenza A virus. Hepatotropic virus titers in blood and livers of AIF-treated, Mx-bearing mice were higher by a factor of 10(3)--10(6) than those in untreated mice of the same genotype, and were comparable to those in genetically susceptible (untreated or AIF-treated) mice. Peritoneal macrophages from Mx-bearing untreated mice were resistant to challenge with a macrophage-adapted strain of influenza A virus even in the presence of AIF. However, when macrophages were taken from resistant mice injected with AIF and also cultivated in the presence of AIF, they were as susceptible to the virus as macrophages taken from susceptible mice. We conclude that interferons is an important factor in resistance to orthomyxoviruses governed by the gene Mx.