With the use of cloned helper-independent Friend leukemia virus (F-MuLV), we have induced a high incidence (approximately 70%) of myelomonocytic leukemia in mice resistant (Fv-6rr or Fv-6rs) to erythroleukemia induction by this virus. The spleen cells from these mice (DBA/2 or BALB/c X DBA/2) were found to contain a high level of progenitor cells capable of forming granulocytic and macrophage colonies (CFU-GM). These CFU-GM, however, were different from those in the spleens of uninfected mice, as they were either very sensitive to or independent of conditioned medium. No erythroid progenitor bursts (BFU-E) or precursor (CFU-E) cells were detected in the spleens of these diseased animals. If these mice with myelomonocytic leukemia were kept alive by transfusion of red blood cells from uninfected mice, tumorigenic cell lines, capable of being transplanted, into adult mice can be isolated. Three such cell lines TTA-1, TTA-3, and TTA-9 have been established, and they retain their morphology of monocytes and macrophages as well as being positive for the monocyte-specific stain alpha-naphthyl-acetate esterase. These myelomonocytic leukemia cell lines can also be induced in culture by spleen cell-conditioned medium to differentiate into macrophages. Other conditioned media such as L-cell-conditioned medium, phytohemagglutinin-stimulated leukocyte-conditioned medium, and WEHI-3 conditioned medium were less effective in their abilities to stimulate differentiation in these myelomonocytic leukemia cell lines.