This invention relates to a hematopoietic stem cell proliferating agent comprising IGF-I, a hematopoietic stem cell proliferating agent comprising IGF-I and at least one protein selected from among SCF, M-CSF, and G-CSF, and a method of growing hematopoietic stem cells which comprises culturing hematopoietic stem cells in a medium containing IGF-I and at least one protein selected from the group consisting of SCF and M-CSF. The hematopoietic stem cell proliferating agent of the invention causes hematopoietic stem cells to proliferate in the undifferentiated state whether in vivo or in vitro and can, therefore, be used for amelioration of the cytopenia induced by radiotherapy or chemotherapy using anticancer drugs, prevention of infectious diseases associated with lymphopenia, or in vitro culture for multiplication of hematopoietic stem cells and extrasomatic culture of recombinant stem cells in gene therapy.