The interaction between p145(c-KIT) and p210(bcr-abl) in transduced cell lines, and the selective outgrowth of normal progenitors during long-term culture of chronic myeloid leukemia (CML) cells on stroma deficient in stem-cell factor (SCF) suggests that the response of CML cells to SCF may be abnormal. We examined the proliferative effect of SCF(100 ng/mL), provided as the sole stimulus, on individual CD34(+) cells from five normal donors and five chronic-phase CML patients. Forty-eight percent of isolated single CML CD34(+) cells proliferated after 6 days of culture to a mean of 18 cells, whereas only 8% of normal CD34(+) cells proliferated (mean number of cells generated was 4). SCF, as a single agent, supported the survival and expansion of colony-forming unit-granulocyte-macrophage (CFU-GM) from CML CD34(+)CD38(+) cells and the more primitive CML CD34(+)CD38(-) cells. These CFU-GM colonies were all bcr-abl positive, showing the specificity of SCF stimulation for the leukemic cell population. Coculture of CML and normal CD34(+) cells showed exclusive growth of Ph+ cells, suggesting that growth in SCF alone is not dependent on secretion of cytokines by CML cells. SCF augmentation of beta1-integrin-mediated adhesion of CML CD34(+) cells to fibronectin was not increased when compared with the effect on normal CD34(+) cells, suggesting that the proliferative and adhesive responses resulting from SCF stimulation are uncoupled. The increased proliferation may contribute to the accumulation of leukemic progenitors, which is a feature of CML.