THE development of blood cells including expansion of megakaryocyte progenitor cells requires the interplay of marrow stromal cells and polypeptide cytokines(1-10). Recently, characterization of c-Mpl, the receptor encoded by the proto-oncogene c-mpl, revealed structural homology with the haematopoietic cytokine receptor family(11), and its involvement in megakaryocyte development(12). We report here that the ligand for c-Mpl(13) is relatively lineage specific, works both alone and synergistically with early acting cytokines to support megakaryocyte colony formation, and acts at a late stage of development to increase megakaryocyte size, polyploidization and expression of differentiation markers. In vivo, c-Mpl ligand stimulates platelet production by greatly expanding marrow and splenic megakaryocytes and their progenitors, and by shifting the distribution of megakaryocyte ploidy to higher values. Thus, as c-Mpl ligand has the expected characteristics of the major regulator of megakaryocyte development, we propose that it be termed thrombopoietin.