Murine erythroblastosis virus (MuEV), also called murine leukemia virus-Kirsten, is a member of the murine type-C-RNA leukemia-sarcoma group of oncogenic viruses. Like other members of this group, MuEV can elicit both a hemolytic disorder and an oncogenic response. Neonatal rats infected with MuEV succumb to this hemolytic disorder unless they are treated with the synthetic double-stranded polyribonucleotide, polyinosinic-polycytidylic acid (poly I-poly C). Animals receiving poly I-poly C had markedly reduced levels of virus reproduction as measured by bioassay and electron microscopy. The proliferation of erythroblasts after MuEV infection in animals not receiving poly I-poly C appeared to be an erythropoietin-dependent compensatory response to hemolysis. The hemolysis itself seemed to require virus reproduction in the cell types affected. Administration of poly I-poly C to MuEV-infected rats inhibited virus reproduction and thus may circumvent the hemolytic disease syndrome. The ultrastructure of the virus and of the virus reproduction was also studied.