The recent cloning of the ligand of the c-MpI hematopoietin receptor has indicated a major role for this cytokine in the development of megakaryocytes. In this study we have applied c-MpI ligand (thrombopoietin [TPO]) in the setting of syngeneic transplantation in an attempt to accelerate the reconstitution of platelets. Donor mice were treated with 20 kilounits (kU)/d TPO intraperitoneally (ip) for 5 days. This resulted in a 2.5-fold increment in platelet counts from 1,119 × 10⁹/L to 2,582 × 10⁹/L (mean, n = 7). Total numbers of hematopoietic progenitor cells in bone marrow (BM) and spleen, as assessed in a colony-forming unit-granulocyte erythroid monocyte macrophage (CFU-GEMM) colony assay (55.3 v38.6 × 10³ CFU/femur; 27.3 v16.3 × 10³ CFU/spleen, mean, n = 7) as well as total numbers of burst-forming unit-erythroid (BFU-E) (24.0 v16.4 × 10³/femur; 10.2 v1.9 × 10³/ spleen, mean, n = 7), were significantly higher in TPO-treated donors than in saline-treated controls. Female Balb-C mice were lethally (8.5 Gy) irradiated and transplantated with 10⁵ BM cells. After transplantation, groups of mice were treated with recombinant murine TPO at a dose of 20 to 30 kU/d ip or subcutaneously (SC) for 5 to 14 days. Using this dose and schedule, TPO did not stimulate the recovery of platelets in comparison with control animals transplanted with equal cell numbers but given vehicle alone. In other experiments, 10⁵ BM cells were procured from TPO-treated donor mice and transplanted into lethally irradiated recipient mice. In comparison with animals transplanted with an equal number of BM cells derived from saline-treated controls, recipients of TPO-treated BM cells had significantly faster platelet recovery and higher platelet nadir counts (88 v30 × 10⁹/L, mean, n = 20). Transplantation of TPO-treated BM cells also resulted in an accelerated recovery of erythrocytes and increased erythrocyte nadir counts (7.2 v 5.0 × 10¹²/L, mean, n = 20). At the day of platelet nadir (day 12 after transplantation) these animals had higher numbers of BFU-Es (770 v422, mean, n = 5) in the marrow and also had higher reticulocyte counts (44°/₀₀ v8°/₀₀, mean, n = 5) in the blood. Therefore, the accelerated recovery of erythrocytes may be a direct effect of TPO on erythropoiesis. Our results indicate that pretreatment of donor animals with TPO before marrow procurement may be an effective alternative method to accelerate the reconstitution of platelets and erythrocytes after lethal irradiation and syngeneic stem cell transplantation.