Thrombopoietin (TPO) is the recently isolated lineage-dominant hematopoietic factor that plays a pivotal role in the regulation of megakaryocytopoiesis and thrombopoiesis. In vivo studies have shown that daily multiple injections of pegylated human megakaryocyte growth and development factor (PEG-rHuMGDF), a truncated molecule related to human TPO, modified with polyethylene glycol, greatly improve thrombocytopenia and in most cases anemia and neutropenia in myelosuppressed animal models. In this study, we further examined various administration protocols of PEG-rHuMGDF on thrombocytopenia in mice treated with a combination of irradiation and carboplatin. After the myelosuppressive treatment on Day 0, mice received the same amount of PEG-rHuMGDF beginning on Day 1 by a single, 3 times (on alternate days), or 7 day daily administration. A single injection of PEG-rHuMGDF significantly reduced the severity and duration of thrombocytopenia and anemia with a concomitant accelerated recovery of megakaryocytic and erythroid progenitors in the bone marrow, similar to the 2 other administration protocols. As the start of a single injection of PEG-rHuMGDF was delayed, its therapeutic effects were attenuated. These results indicate that an administration of PEG-rHuMGDF at an earlier time after the myelosuppressive treatment is necessary to improve thrombocytopenia and anemia.