It is intended to provide agents promoting the proliferation and/or differentiation of hematopoietic stem cells and/or hematopoietic precursor cells which are useful as remedies for diseases induced by the insufficient proliferation and/or differentiation of hematopoietic stem cells and/or hematopoietic precursor cells, in particular, diseases associated with a diffuse decrease in hematopoietic cells and/or hematopoietic hypofunction. The above problem is solved by providing agents for promoting the proliferation and/or differentiation of hematopoietic stem cells and/or hematopoietic precursor cells which contain as the active ingredient cofilin.