CD34(+)-selected hematopoietic progenitor cells are being increasingly used for autotransplantation, and recent evidence indicates that these cells can be expanded ex vivo. Of 15 patients with solid tumors undergoing a phase I/II clinical trial using CD34(+)-selected peripheral blood progenitor cells (PBPCs) after high-dose chemotherapy, we analyzed the frequency of long-term culture-initiating cells (LTCIC) as a measure of transplantation potential before and after ex vivo expansion of CD34+ cells. PBPCs were mobilized by combination chemotherapy and granulocyte colony-stimulating factor (G-CSF). The original unseparated leukapheresis preparations, the CD34(+)-enriched transplants, as well as nonabsorbed fractions eluting from the CD34 immunoaffinity columns (Ceprate; CellPro, Bothell, WA) were monitored for their capacity to repopulate irradiated allogeneic stroma in human long-term bone marrow cultures. We found preservation of more than three quarters of fully functional LTCIC in the CD34(+)-selected fractions. Quantitation of LTCIC by limiting dilution analysis showed a 53-fold enrichment of LTCIC from 1/9,075 in the unseparated cells to an incidence of 1/169 in the CD34+ fractions. Thus, in a single apheresis, it was possible to harvest a median of 1.65 x 10(4) LTCIC per kg body weight (range, 0.71 to 3.72). In addition, in six patients, large-scale ex vivo expansions were performed using a five-factor cytokine combination consisting of stem cell factor (SCF), interleukin-1 (IL-1), IL-3, IL-6, and erythropoietin (EPO), previously shown to expand committed progenitor cells. LTCIC were preserved, but not expanded during the culture period. Optimization of ex vivo expansion growth factor requirements using limiting dilution assays for LTCIC estimation indicated that the five-factor combination using SCF, IL-1, IL-3, IL-6, and EPO together with autologous plasma was the most reliable combination securing both high progenitor yield and, at the same time, optimal preservation of LTCIC. Our data suggest that ex vivo-expanded CD34+ PBPCs might be able to allow long-term reconstitution of hematopoiesis.