Plasma obtained from rabbits made thrombocytopenic by the injection of antiplatelet serum (APS) stimulated megakaryocyte growth in an in vitro bone marrow culture. Although the number of megakaryocytes that grew was increased on average to 136%, the major effect was on megakaryocyte ploidy, which showed a shift in the modal ploidy class from 8N to 16N, an average change in the geometric mean ploidy from 8.7N to 11.8N, and the appearance of some 64N megakaryocytes. This in vitro stimulation of megakaryocyte number and ploidy was shown to be due to the appearance in the circulation of a positive activity, which we have named megapoietin. Levels of megapoietin in thrombocytopenic plasma could be quantitated by measuring the extent of megakaryocyte ploidization in vitro. The relationship over time of megapoietin to changes in the platelet count was then assessed in a series of rabbits made thrombocytopenic by APS injection. Although not elevated after 3 hours of thrombocytopenia, megapoietin increased after 8 hours to 48% of the maximal level, which occurred after 24 hours of thrombocytopenia. Levels of megapoietin were inversely and proportionally related to the platelet count during thrombocytopenia and the subsequent rebound thrombocytosis. During persistent thrombocytopenia, megapoietin levels remained maximally elevated. These results suggest that megapoietin may play a physiologic role in the feedback loop between the platelet and the bone marrow megakaryocytes.