Interleukin-11 (IL-11) is a newly identified lymphohematopoietic cytokine originally derived from the primate bone marrow stromal cell line, PU-34. Separately, we reported that IL-11 augments IL-3-dependent proliferation of primitive murine hematopoietic progenitors in culture. We have now examined the synergistic interactions between IL-11 and IL- 4 in support of colony formation from marrow cells of mice treated 2 days before with 150 mg/kg 5-fluorouracil. Neither recombinant human IL- 11 nor murine IL-4 alone was effective in the support of colony formation. When the two factors were combined, there was major enhancement of colony formation, including that of multilineage colony- forming cells. Serial observations (mapping studies) of development of multipotential blast cell colonies indicated that the synergy between IL-11 and IL-4 is due in part to shortening of the dormant period of the stem cells, an effect very similar to that of IL-6 and granulocyte colony-stimulating factor. The combination of IL-11 and IL-4 may be useful in the stimulation of dormant hematopoietic stem cells in vivo.