Murine long-term bone marrow cultures that support myelocyte cell development were infected at initiation with a recombinant retrovirus containing v-abl and neo genes. Other cultures were uninfected controls. Harvested nonadherent cells were tested weekly for granulocyte macrophage colony formation (CFU-GM) , factor independent colony formation and anti-G418 CFU-GM. Cultures infected by virus with v-abl demonstrated a significant increase in cumulative nonadherent cell and CFU-GM in first 1 ~2 week cultures. There was anti-G418 CFU-GM, and factor independent colony formation in the supernatant nonadherent cells in the infected cultures. Cells from control cultures did not form anti-G418 CFU-GM and factor independent colonies. The data supported the conclusion that v-abl expression in long-term marrow cultures stimulated hematopoietic stem cell proliferation and differentiation. Key words: oncogene; recombined retrovirus; cell culture