Radiation chimeras produced by crosswise transfers of bone-marrow cell among histocompatible mice susceptible, or genetically resistant, to lethal challenge by a number of myxoviruses were used to test whether macrophage resistance (as assessed in vitro) and resistance of the animal (as measured in vivo), both previously shown to be brought about by the gene Mx, were causally related. 49 chimeras were tested individually, both of resistance of their macrophages to in vitro challenge with M-TUR (a strain of avian influenza virus A/Turkey/England/63 adapted to grow in cultured mouse peritoneal macrophages), and for resistance of the animal in vivo upon challenge with pneumotropic, neurotropic, or hepatotropic influenza viruses. Cultivated Kupffer cells and peritoneal macrophages harvested from chimeric mice expressed the resistance phenotype of the bone-marrow donor irrespective of the host environment in which they had differentiated. However, susceptibility or resistance in vivo was according to the genotype of the host. Thus, inborn resistance of radiation chimeras was found to be independent of Mx-gene expression in cells of the hemopoietic system.