We report that acetyl-N-Ser-Asp-Lys-Pro (AcSDKP), which removes progenitor cells from cell cycle, in combination with granulocyte-colony stimulating factor (G-CSF) can significantly improve myelorestoration following irradiation (7 Gy). Peripheral blood, spleen and bone marrow (BM) cell recovery and progenitor cell reconstitution [IL-3-responsive colony-forming cells (CFC) and high proliferative potential colony-forming cells (HPP-CFC)] were studied. Studies on the optimal schedule of AcSDKP administration revealed maximal effects on progenitor cells when AcSDKP was administered as a continuous infusion for 3 d starting 24 h prior to irradiation and used in combination with G-CSF. The numbers of CFC and HPP-CFC in the BM were significantly increased following irradiation in mice receiving AcSDKP and G-CSF as compared to either drug alone. The numbers of CFC in the spleen were significantly increased in mice receiving AcSDKP and G-CSF on days 10 and 14 as compared to AcSDKP alone, but not G-CSF. Similarly, CFC and HPP-CFC in the spleen were significantly increased in mice receiving AcSDKP and G-CSF on day 18 as compared to mice receiving PBS and G-CSF. These studies suggest that AcSDKP in combination with G-CSF may have potential for the protection of progenitor cells in patients undergoing intensive chemo- and/or radiotherapy.