Megakaryocytes undergo changes in ploidy in vivo in response to varying demands for platelets. Attempts to study the putative factor(s) regulating these ploidy changes have been frustrated by the lack of an appropriate in vitro model of megakaryocyte endomitosis. This report describes a culture system in which rat bone marrow is depleted of identifiable megakaryocytes and enriched in their precursor cells. Morphologically identifiable megakaryocytes appear when the depleted marrow is cultured in vitro. The total number of nucleated cells, as well as the number of megakaryocytes and their ploidy distribution, are quantitated very precisely by flow cytometry. Although the total number of nucleated cells declines by 35% to 40% over 3 days in culture, the number of megakaryocytes rises 10-fold. The number of nucleated cells, the number of megakaryocytes, and the extent of megakaryocyte ploidization behave as independent variables in culture and are dependent on the culture conditions. The addition of recombinant erythropoietin promotes a rise in the number of megakaryocytes and a shift in ploidy to higher values while recombinant murine granulocyte-macrophage colony stimulating factor is without effect on the cultured megakaryocytes. This in vitro system may provide a means to study those factors that affect megakaryocyte growth and ploidization.