A helper-independent Friend leukemia virus was used to infect bone marrow cultures. This virus induces myeloblastic leukemia in mice after a long latency period. Infection of the bone marrow cultures resulted in the in vitro production of myeloblastic leukemogenesis after a long latency period. Three steps were observed in the evolution of the infected cultures, and permanent cell lines were derived at each step. This allowed us to individualize three successive events in the course of the myeloblastic transformation: (i) an abnormal responsiveness to the physiological hormone granulo-macrophagic colony-stimulating factor, (ii) the acquisition of growth autonomy, and (iii) the acquisition of in vivo tumorigenicity.