Summary Three growth factors present in platelets, namely platelet-derived growth factor (PDGF), transforming growth factor-β (TGF-β) and epidermal growth factor (EGF), have been implicated in the pathogenesis of bone marrow fibrosis frequently associated with myeloproliferative disorders. In this study, regulation of the proliferation, as well as collagen and fibronectin synthesis from marrow fibroblasts by TGF-β was investigated. TGF-β alone at high plating density stimulated the proliferation of cells at low concentrations, but rather showed inhibition at high concentrations in both MPD patients and control subjects. In the presence of PDGF, which has been confirmed to be a main growth factor for marrow fibroblasts, low concentration of TGF-β inhibited the proliferation at low cell density, but there was no inhibition at high cell density. The synthesis of both type I and type III procollagen was enhanced by high concentrations of TGF-β in both MPD patients and control subjects, while PDGF or EGF showed no effect. The fibronectin synthesis was also enhanced by TGF-β, but not by PDGF or EGF. These results suggest that growth and stromal protein synthesis of fibroblasts causing marrow fibrosis are regulated by TGF-β as well as PDGF and EGF, when these factors are released or leaked from platelets or megakaryocytes into marrow environment in MPD patients.