AbstractFibroblast growth factor 9 (FGF-9), a novel member of the FGF family, was found to have thrombopoietic activityin vitroandin vivo.In anin vitromegakaryocyte colony-stimulating factor assay, anti-mouse interleukin-6 (IL-6) monoclonal antibody neutralized FGF-9 activity. This suggests that the activity may be exerted via IL-6 induction. BALB/c mice that received subcutaneous FGF-9 injections of 4 to 100μg/day for 2 weeks showed a dose-dependent transient increase in peripheral platelet counts 10 to 12 days after the first treatment. Histologic studies showed a marked increase in megakaryocytes in the bone marrow and extramedullary hematopoiesis in the spleen and the liver. Examination of changes in the DNA content of bone marrow megakaryocytes revealed that the ploidy distribution underwent a marked shift 3 days after FGF-9 injection, with a large increase in the 2N megakaryocyte population. The major modal ploidy shifted from the normal 16N to 2N. The number of megakaryocyte progenitor cells in FGF-9-treated mice increased up to 1.5-fold in the bone marrow and 10-fold in the spleen on day 6. These results indicate that FGF-9 acts on thein vivoproliferation of megakaryocytes.