Abstract: There is disclosed a chimeric mammal having a stable bone marrow graft of human hematopoietic cells capable of differentiating into multiple lineages of human mature cells, wherein at least 30% of the hematopoietic cells in the mammal's bone marrow are of human origin. The inventive method comprises sublethally irradiating an immunodeficient mammal, infusing human hematopoietic cells into the mammal and administering an effective amount of human mast cell growth factor (MGF) and a human granulocyte macrophage colony stimulating factor/interleukin-3 fusion protein (GM-CSF/IL-3 FP) to promote engraftment of human cells within the chimeric mammal's bone marrow.