To elucidate the role of recombinant human colony-stimulating factors (CSFs) for expanding peripheral blood (PB) CD34+ cells, these cells were purified up to 94.5% +/- 1.3% and the effects of individual and combined CSFs on the proliferation and differentiation of these cells were studied in a 7-day suspension culture. The majority of CD34+ cells coexpressed CD38 (81.8% +/- 5.1%), but was negative for CD33 (88.5% +/- 3.4%). Among the individual CSFs examined, recombinant interleukin-3 (rIL-3) was identified as the most potent factor for expanding PB progenitor cells and increased nonerythroid progenitor cells 13- +/- 4- fold (P < .01). Recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF), recombinant granulocyte-CSF (rG-CSF), recombinant macrophage-CSF (rM-CSF), rIL-6, rIL-11, and recombinant stem cell factor (rSCF) did not alone expand nonerythroid progenitor cells. A combination of 5 CSFs, ie, rIL-3, rIL-6, rGM-CSF, rG-CSF, and rSCF, was identified as the most potent combination of those tested and increased nonerythroid progenitor cells 57- +/- 11-fold. After a 7-day suspension culture of CD34+ cells with these 5 CSFs, CD34+ cells expanded 14.5- fold, and CD34+/CD33- cells and CD34+/CD33+ cells were also expanded 2.9-fold and 307-fold, respectively. Most secondary colonies derived from expanded cells were small; however, the absolute number of large- sized colonies expanded 5.9- +/- 3.3-fold. Thus, the combination of CSFs can achieve a degree of amplification of PB CD34+ cells. The capability of in vitro expansion of PB CD34+ cells as an adjunct to PB stem cell transplantation is worthy of consideration.