We isolated erythroleukemic cell lines arrested at different levels of the erythroid differentiation pathway. One cell line (CB5), established from mice infected with the helper-independent Friend murine leukemia virus (F-MuLV), exhibited properties similar to those of the normal erythroid progenitor burst-forming cell (BFU-E). Six erythroleukemic cell lines, which were established from the anemia-inducing Friend virus complex (FV-A)-infected mice, formed erythroid colonies similar to the erythroid colony-forming precursor cell (CFU-E) after induction with dimethyl sulfoxide or erythropoietin. Three lines that were established from the polycythemia-inducing Friend virus complex (FV-P)-infected mice also formed low proportions (2-5%) of CFU-E-like colonies after induction by these same inducers. These data, together with the earlier findings that F-MuLV induces an increase in the levels of BFU-E and that FV-A or FV-P stimulates enhancement of CFU-E early after infection, indicate that the erythroleukemic cell lines isolated late in the diseases are at the same levels of differentiation as the leukemic cells in the corresponding initial stages. These cell lines with properties of BFU-E and CFU-E can be induced to differentiate in culture and should add to our understanding of the nature of erythroid progenitor cells and their early differentiation programs.