The effect of interleukin-11 (IL-11), alone and in combination with other hematopoietic growth factors, was studied in liquid suspension cultures of murine bone marrow. Stem and progenitor cell contents of input bone marrow 2 days after 5-fluorouracil (5-FU) treatment (FU2dBM) and lineage-depleted, Sca-1-positive FU2dBM (Lin-Sca+Fu2dBM) were compared with output values obtained after 6 days in different growth factor combinations. Stem and progenitor cell recoveries were measured by long-term in vivo competitive repopulation, spleen colony-forming unit (CFU-S), and in vitro limiting dilution long-term bone marrow culture (cobblestone area-forming cell [CAFC]) assays. IL-11, IL-3, IL-6, and Steel factor (SF) were tested separately and in combinations of two, three, and four factors. IL-11 was incapable of maintaining the survival of stem or progenitor cells as a single agent but synergized with either IL-3 or SF to expand progenitor cell output in cultures of FU2dBM. IL-11 greatly enhanced progenitor cell expansion from FU2dBM when added to two- and three-factor combinations of SF, IL-3, and IL-6. The recovery of long-term repopulating ability was enhanced four-fold by incubation in SF plus IL-11 compared to SF alone. IL-11 in combination with SF, IL-3, and IL-6 stimulated a 24,000-fold expansion of progenitor cells from stem cell-enriched Lin-Sca+FU2dBM. Overall, the results indicate that IL-11 is a potent synergistic factor for stem cell proliferation and expansion of progenitors in liquid culture.