BACKGROUND Ex vivo expansion of progentior cells may shorten hematopoietic regeneration after myeloablative chemoradiotherapy, increase target cells for gene therapy, and improve purging of progenitor cell components. METHODS Marrow cells were incubated for 1 week in suspension culture with and without IL-10, IL-3, and SCF. As long-term culture initiating cells (LTC-ICs) represent early hematopoietic progenitors in vitro, these cells were quantified at initiation and after a 1-week culture period in a limiting dilution assays. Additionally, immunophenotyping of cells before and after culture was performed. RESULTS In six experiments, marrow cells cultured for 1 week with IL-10, IL-3, and SCF showed a significant increase (almost doubling) in LTC-ICs as compared with marrow cells before expansion. Additionally, an increased proliferative capacity of LTC-ICs was achieved with a sevenfold increase of committed colony-forming cells and a 10-fold proliferation of high proliferative potential colony-forming cells. Immunophenotyping revealed a sevenfold increase of CD34+ CD45 RA- cells in IL-10-, IL-3-, SCF-stimulated suspension cultures. In unstimulated cultures, no LTC-ICs were maintained after 1 week. CONCLUSIONS Expansion of LTC-ICs by IL-10, IL-3, and SCF has not been shown so far. This in vitro model allows expansion of LTC-IC if compared with the input of progenitor cells without extensive progenitor cell manipulation. This should be an attractive model for in vitro purging, gene transfer, or expansion of progenitor cells to allow rapid engraftment after myeloablative chemotherapy.