The effect of the recently cloned cytokine bone morphogenetic protein 9 (BMP-9) on colony formation and generation in vitro clonable hematopoietic progenitors (CFU-C) in serum-free liquid cultures (LC) of both normal and post-5-fluorouracil murine bone marrow cells was studied in the presence of various other cytokines. In LC, BMP-9 concentrations of 100 ng or more per ml led to complete inhibition of Steel Factor (SF) + interleukin-11 (IL-11) or IL-12 supported CFU-C generation, which was partly abrogated when IL-3 was additionally included. We found this inhibitory effect of BMP-9 to be mediated by an increased TGF-beta1 elaboration and TGF-beta1 mRNA expression in bone marrow cells with increasing BMP-9 concentrations. In the presence of neutralizing antibodies (Ab) against TGF-beta1, BMP-9 concentrations of 3 ng or higher synergized with IL-3, SF+IL-3, SF+IL-11/12, or IL-3+SF+IL-11/12 to increase CFU-C generation. Similarly, high BMP-9 concentrations dramatically inhibited primary colony formation induced by SF+IL-11/12, whereas in the presence of TGF-beta1 neutralizing Ab only 3 ng or more BMP-9 per ml stimulated both the time of colony appearance, the colony size and colony numbers in the presence of IL-3, M-CSF, GM-CSF, SF, SF+Flt3-L, SF+IL-3, SF+IL-11/12 or IL-3+SF+IL-11/12. BMP-9 neither stimulated CFU-C generation nor colony formation as a single factor, nor did it synergize with thrombopoietin (Tpo), erythropoietin (Epo), Flt3-L, IL-11, IL-12 or G-CSF. The effect of BMP-9 on its target cells was direct as demonstrated using single-sorted stem cells. These observations demonstrate that BMP-9 plays a dual role in regulating proliferation of primitive hemopoietic progenitor cells. Thus, in addition to its ability to enhance TGF-beta1 elaboration in bone marrow cells, it acts as a potent synergistic activity that is different from SF, Flt3-L, IL-11 or IL-12. BMP-9 mRNA was exclusively detected in the liver of adult mice, whilst no expression was found in stromal cell lines propagated from day-16 fetal liver or neonatal or adult bone marrow. 125I-BMP-9 bound specifically to a high percentage of blast cells in lineage-depleted post-fluorouracil bone marrow cells and to megakaryocytes in normal and post-fluorouracil bone marrow, indicating that BMP-9R are expressed on these cells. The dissociation between the site of BMP-9 production and its target cells in the bone marrow makes BMP-9 a hemopoietic hormone.