Bone marrow cells from a patient with acute myeloblastic leukemia were simultaneously cultured in vitro under conditions that favored the survival of either (1) leukemic progenitors (leukemic suspension culture), or (2) normal progenitors (long-term bone marrow culture). Whereas cells that were morphologically primitive and cytochemically leukemic persisted in leukemic suspension culture, they were progressively and completely replaced in long-term bone marrow culture by neutrophilic granulocytes and subsequently by macrophages. However, Auer rods were present in the maturing myeloid cells, including polymorphonuclear neutrophils, between the 7th and 30th days of long-term bone marrow culture, indicating that they were derived directly from the original leukemic population. This observation suggests that, at least in some patients, selection of cells with the potential for terminal differentiation may be the underlying mechanism responsible for the purging properties that have been attributed to long-term bone marrow culture.