Abstract: The present inventors discovered that the administration of an agonistic minibody (VB22B sc(Fv)2) against the TPO receptor resulted in not only the induction of human megakaryocyte-specific differentiation (increase in platelet precursor cells), but also the engraftment of transplanted hematopoietic stem cells derived from human cord blood (CD34-positive cells) and significant increase in multi-lineage hematopoietic precursor cells. TPO and TPO receptor agonists can be used as agents for promoting the growth of CD34-positive hematopoietic cells or agents for promoting the engraftment of transplanted cells in the bone marrow, which can be effective when administered alone (without using G-CSF and erythropoietin in combination) after hematopoietic stem cell transplantation (in particular, cord blood transplantation).