This invention has as its object providing promoters of the growth and/or differentiation of hematopoietic stem cells and/or hematopoietic progenitors which are useful as therapeutics of diseases that result from insufficient growth and/or differentiation of hematopoietic stem cells and/or hematopoietic progenitors, in particular, as therapeutics of panhematopenia and/or diseases that are accompanied by hematopoietic hypofunction. The invention attains the stated object by providing promoters of the growth and/or differentiation of hematopoietic stem cells and/or hematopoietic progenitors that contain Cofilin as an active ingredient.