Integrated Friend murine leukemia virus copies were analyzed by the Southern blotting procedure in myeloblastic cell lines obtained after in vitro infection of long-term mouse bone marrow cultures. Several steps leading to the generation of malignant myeloblastic cells after a long latency period were observed in the evolution of infected cultures. Shortly after infection, a random distribution of integrated provirus copies was observed in the DNA of normally differentiating myeloid cells. In contrast, a distinct pattern of integrated Friend murine leukemia virus copies was evident in the first non-differentiating immature myeloblastic cells appearing in cultures, suggesting a monoclonal origin of these cells. For each cell line, characteristic hybridizing fragments were conserved during the 1-year culture period necessary for the acquisition of tumorigenic properties and were also observed in tumors grafted in vivo. We can conclude that monoclonality is effective very early in the myeloid transformation process, as soon as the precursor cells are blocked in their differentiation.