Friend erythroleukemia virus (FLVj is a complex of two viruses, a defective spleen focus virus component (SFFV) and a helper independent virus, F-MuLV. The virus complex induces in susceptible mice a multistage disease, an early stage (1-3 weeks) of rapid erythropoietic changes and a late stage, from which erythroleukemic cell lines (FLC) can be established. While the F-MuLV alone does not induce any detectable changes in adult mice, it has been found recently to induce, in newborn BALB/c and NIH/Swiss mice, splenomegaly and certain early erythropoietic changes. The present study indicates that if the infected mice were kept alive by red cell transfusion and the cells from these F-MuLV-induced enlarged spleens were transplanted repeatedly in adult mice, erythroid tumorigenic colonies (CFU-FV) can be detected. These helper virus of induced CFU-FV (CFU-FV-H) have properties that differed from those CFU-FVs induced by the FLV complexes. The colonies are more diffuse in morphology and do not release any detectable SFFV activities. In addition, FLC lines can also be established from these F-MuLV infected spleens and CFU-FV-H and have been in culture for eight months. They are found to be distinct from those established from CFU-FV induced by the FLV complexes. First, these FLC lines are more selective in the type of medium they can grow in, for while FLC lines induced by FLV complexes will grow in both a-medium and lscove medium, the FLC induced by F-MuLV will only grow in lscove medium and not in a-medium.