The treatment of acute nonlymphocytic leukemia results in predictable bone marrow hypoplasia and eventual cellular repopulation. In order to study this postchemotherapy repopulation, assays for hematopoietic progenitor cells were performed on bone marrow samples obtained from seven patients with acute nonlymphocytic leukemia who had received similar chemotherapeutic induction regimens. Burst-forming units (erythrocyte), colony-forming units (megakaryocyte), colony-forming units (granulocyte-macrophage), and colony-forming units (granulocyte-erythrocyte-megakaryocyte-macrophage) were cloned from human bone marrow mononuclear cells 5 and/or 10 days following completion of chemotherapy. All patients were pancytopenic and had hypocellular marrows when studied. Assays were performed 7 to 30 days prior to complete remission. Colony-forming units (granulocyte-macrophage) were equivalent to control values 5 days following chemotherapy, while burst-forming units (erythrocyte) and colony-forming units (granulocyte-erythrocyte-megakaryocyte-macrophage) were not assayable at that time. Ten days following chemotherapy, colony-forming units (granulocyte-erythrocyte-megakaryocyte-macrophage) and colony-forming units (granulocyte-macrophage) were 200 and 250% of normal controls, respectively, while burst-forming units (erythrocyte) were 29% of control values. Colony-forming units (macrophage) were 10 to 15 times normal values 10 days following chemotherapy. In contrast to colonies from normal individuals, those grown from marrow obtained following chemotherapy were frequently macroscopic and were composed of thousands of cells. Patient marrow had larger proportions of progenitor cells in S phase of the cell cycle than did normal controls. These studies suggest the presence of a stem cell in human bone marrow which is resistant to chemotherapeutic agents and has a high capacity to regenerate hematopoietic progenitor cells. The period following completion of chemotherapy for acute nonlymphocytic leukemia appears suitable for the study of the hierarchical nature of human hematopoiesis.