Objective: We investigated whether primary chronic myelogenous leukemia (CML) hematopoietic progenitors demonstrated altered proliferation and maturation in response to growth factor (GF) stimulation. Methods: The effect of GF stimulation on proliferation and expansion of committed and primitive progenitors (colony forming cells [CFC]) was evaluated. Results: Culture of CML and normal CD34+ cells with different GF for 7 days resulted in similar expansion of committed progenitors (CFC). In contrast, GF culture conditions that expanded normal primitive progenitors (week-6 long-term culture-initiating cells (LTC-IC)] led to depletion of CML LTC-IC numbers. GF culture also resulted in increased depletion of week-10 extended LTC-IC, which represent an even more primitive progenitor population, from CML compared with normal CD34+ cells. CML CD34+ cells enter into cycle more quickly than normal CD34+ cells and CML CFC expansion was accelerated compared to normal CFC. Evaluation of primitive progenitor proliferation using PKH-26 and single-cell LTC-IC analysis demonstrated that the majority of CML LTC-IC remaining after GF culture originated from divided CD34+ cells, whereas GF-cultured normal LTC-IC were derived mainly from undivided cells. Depletion of CML primitive progenitor numbers in association with increased proliferation suggests increased sensitivity to GF-induced maturation. Conclusions: These studies indicate that CML primitive progenitors have enhanced sensitivity to GF-induced cell division and maturation. Altered GF responsiveness may contribute to abnormal expansion of malignant myeloid cells in CML. These findings may also be applied toward the development of novel approaches to select benign stem cells in CML. Copyright © 2000 International Society for Experimental Hematology.