Adenoviral vectors may be useful tools to deliver a cytokine in vivo. A single intravenous injection of an adenovirus vector containing the human thrombopoietin (TPO) cDNA (AdRSVhuTPO) was able to induce a thrombocytosis for more than 6 weeks in SCID mice, associated with a megakaryocyte (MK) hyperplasia in different organs. A marrow and spleen fibrosis was observed at 6 weeks. In immunocompetent mice, a single AdRSVhuTPO injection led to a moderate and transient thrombocytosis without myelofibrosis. To evaluate the usefulness of TPO for the prevention of secondary side-effects during an aplastic period, mice were subjected to a myeloablative regimen 7 days after the intravenous AdRSVhuTPO injection. In this setting, TPO prevented mortality by accelerating hematological recovery. Survival was essentially related to an improvement in the leukopenia since all control mice died from septicemia. However, the effects of TPO may be potentiated by the release of inflammatory cytokines following the adenovirus infection; AdRSV beta galactosidase injected-mice had higher numbers of BFU-E and CFU-GM in the marrow than PBS-injected mice. Myelosuppression induced transient immunosuppression responsible for a sustained expression and elevation of platelet numbers for at least 5 months. These results further suggest that TPO may be an effective therapy in diminishing hematological complications related to myeloablative regimens, but emphasize that immunosuppression secondary to myelosuppression may lead to sustained expression associated with a risk of thrombosis and myelofibrosis when delivered by adenovirus vectors.