Murine interleukin 9 (mIL-9) is a T-cell-derived growth factor that stimulates erythroid burst-forming units (BFU-E) from murine bone marrow. We further investigated this activity using enriched mouse bone marrow progenitors and the multipotent interleukin 3 (IL-3)-dependent FDCP-Mix cell line. We report here that mIL-9 stimulates erythroid burst formation of total bone marrow cells and accessory cell-depleted bone marrow cells, even in serum-free cultures. On the other hand, we observed that although mIL-9 could not support proliferation of FDCP-Mix cells, it favors erythroid differentiation of these cells in the presence of both IL-3 and erythropoietin. These results strongly suggest that mIL-9 acts directly on mouse erythroid progenitor cells.