Characterization of primitive subpopulations of normal and leukemic cells present in the blood of patients with newly diagnosed as well as established chronic myeloid leukemia, Blood 88

positive (Ph+) progenitors, including long-term culture-ini-tiating cells (LTC-IC) as well as colony-forming cells (CFC), have been previously described in the blood of patients with chronic myeloid leukemia (CML) in chronic phase with high white blood cell counts. In the present study, which focused primarily on an analysis of circulating progenitors present in such patients at diagnosis, we discovered the frequent and occasionally exclusive presence of circulating normal (Ph-1 LTC-IC, often at levels above those seen for LTC-IC in the blood of normal individuals. The presence of detectable numbers of circulating Ph- LTC-IC was independent of the fact that the same peripheral blood samples also contained elevated numbers of predominantly or exclusively Ph+ CFC. Interestingly, both the Ph ' and Ph- LTC-IC in these samples were CD34+CD71- and variably CD38- and Thy-l+, as pre