Abelson murine leukemia virus (A-MuLV) and Harvey murine sarcoma virus (Ha-MSV) are retroviruses carrying unrelated onc genes. However, both of these viruses are capable of stimulating the growth and differentiation of erythroid precursor cells; the target cells for both appear at the same time during fetal development and follow a similar pattern throughout ontogeny. In addition, the colonies induced by each virus are morphologically similar and synthesize the adult form of hemoglobin. However, A-MuLV-infected cells are Epo-independent, whereas Ha-MSV-infected cells are Epo-dependent. Superinfection of Ha-MSV-infected cells with A-MuLV overrides their Epo-dependency. Thus, the consequences of the infection are determined by the interaction of the different onc gene products with identical or similar erythroid cells.