A recombinant fusion protein derived from the coding sequences of two growth factors, interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Milodistim exhibits greater receptor binding affinity and colony stimulating activity than its parent cytokines. This agent stimulates proliferation of immature hematopoietic cells and allows the stimulation and expansion of multi-lineage hematopoiesis from immature bone marrow progenitor cells.