Megakaryocytopoiesis and platelet production can be assessed with reasonable accuracy by quantitative and functional analyses of circulating platelets. The evaluation of megakaryocytopoiesis in culture has remained unsatisfactory, particularly because platelet production is rarely observed. In mouse culture systems, megakaryocytes have been identified almost entirely by measurements of acetyl cholinesterase, size, and ploidy without concomitant assessment of maturation based on such criteria as the formation of granules, demarcation membranes, and cytoplasmic fragmentation. The availability of various thrombopoietic cytokines, in particular thrombopoietin (TPO), and their imminent clinical use has made a more detailed understanding of their effect on differentiation and maturation of the MK lineage more urgent. Therefore, ultrastructural analyses were performed on megakaryocyte-depleted serum-free mouse bone marrow cultures in the presence of TPO alone, TPO plus other cytokines, or under conditions in which TPO and/or other cytokines were blocked with neutralizing agents. These studies show that, while cytokines that use the gp130 receptor subunit may function synergistically with TPO, in the absence of TPO, such cultures do not yield morphologically recognizable MK. On the other hand, TPO alone is able to drive MK to full maturation as evidenced by the generation of granules, demarcation membranes, and cytoplasmic fragmentation into platelets.