Thrombopoietin (TPO), the specific cytokine that regulates platelet production, is expressed in human bone marrow (BM), kidney, and liver. There appears to be no regulation of TPO in the kidney and liver, but TPO messenger RNA (mRNA) expression can be modulated in the stromal cells of the BM. In this study, we used primary human BM stromal cells as a model to study the regulation of TPO mRNA expression in response to various platelet -granular proteins. We showed that platelet-derived growth factor (PDGF) BB and fibroblast growth factor (FGF) 2 stimulated TPO mRNA expression in both a dose-dependent and time-dependent manner. The addition of 50 ng/mL of PDGF and 20 ng/mL of FGF resulted in maximal induction of TPO mRNA expression in 4 hours. We also found that platelet factor 4 (PF4), thrombospondin (TSP), and transforming growth factor-beta (TGF-β) are negative modulators of megakaryocytopoiesis. We observed suppression in TPO mRNA expression with 1 μg/mL of both PF4 and TSP and 50 ng/mL of TGF-β, with maximal suppression occurring 4 hours after the addition of these proteins. Finally, the addition of whole-platelet lysate produced a dose-dependent inhibition of TPO expression. On the basis of these findings, we propose that the platelet -granular proteins studied may regulate TPO gene expression in BM stromal cells by means of a feedback mechanism.