The inbred laboratory mouse strain A2G carries a functional, interferon type 1 inducible gene, Mx which upon expression confers specific resistance to an otherwise lethal dose of influenza virus. We investigated in vivo Mx gene expression by performing Northern hybridisation and in-situ hybridisation on A2G (Mx+) and (CBA/J×C57)F1 (Mx−) mice that were induced either with human, natural interferon; human, recombinant interferon or double stranded poly(I):(C). All 3 inducers were able to stimulate Mx expression in all organs examined in the A2G strain. However, contrary to previous reports, Mx expression was confined to a small number of cell types; the main contributor was most probably mononuclear cells. Specialised cells such as hepatocyte, nephron, ovarian follicle and seminiferous tubules did not show detectable Mx level. There was also constitutive Mx expression in the epithelia of uterus and duodenum which suggested direct gene activation independent of blood-bourne interferon.