A liquid culture technique was used to study regulation of human megakaryocytopoiesis in vitro. Low-density cells from adult bone marrow were cultured in the presence of normal plasma, plasma from patients with aplastic marrows (AP), recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) and interleukin-3 (IL-3). Megakaryocytes (MK) were studied at day 10 of culture by a two-color staining technique using a pool of monoclonal antibodies for their identification and propidium iodide to label DNA. Their ploidy distribution was analyzed by flow cytometry. In some experiments cytoplasmic maturation was also studied by ultrastructural techniques. Normal plasma provides a low number of MK with a ploidy distribution including 8 N and 16 N MK. AP promoted in a dose-dependent manner proliferation of MK and some batches favored endoreplication. This effect was clearly demonstrated when ploidy distribution was compared between normal plasma and AP on parallel marrow cultures. However, ploidy distribution was shifted toward low values compared with uncultured MK. rhGM-CSF had no significant effect on these two parameters. In contrast, rhIL-3 from 0.1 U/mL to 100 U/mL had a proliferative effect but was unable to induce endoreplication. Furthermore, when associated with AP it totally abrogated the effect of AP on endoreplication because in most experiments more than 90% of MK were 2 N and 4 N. This effect was also observed when rhIL-3 was added after 7 days of culture (when it has little proliferative effects). Studies of the maturation of MK grown with rhIL-3 indicate that the majority were small mature cells synthesizing alpha-granules and demarcation membranes. The effect of AP on MK proliferation and endoreplication was not related to IL-6 because its IL-6 content was identical to that of normal plasma and its neutralization did not modify these parameters. In conclusion, this study indicates that liquid culture technique in association with flow cytometry could be a powerful tool in identifying the humoral regulators of human megakaryocytopoiesis.