In this paper, the in vitro growth of bone marrow early (megakaryocyte burst-forming units, BFU-meg) and late (megakaryocyte colony-forming units, CFU-meg) progenitors was evaluated in 18 essential thrombocythemia (ET) patients and 22 normal control subjects. BFU-meg clonality was demonstrated both in normal and ET bone marrows, cultivating these primitive progenitors at limiting dilutions in plasma clot assay: 1 to 7 BFU-meg/2.5 x 10(4) mononuclear non-adherent cells were observed, with a strong correlation in ET [r = 0.955 stimulated by recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) plus recombinant human interleukin (rhIL) 3], as well as in normal controls (r = 0.969). In order to clearly elucidate the in vitro response of ET megakaryocyte (meg) progenitors to recombinant growth factors, the interference of accessory cells (i.e., monocytes, T lymphocytes, and natural killer cells) and human serum were avoided by performing experiments on CD34+ cells in a serum-free fibrin clot assay. The number of both early and late meg progenitors in ET was significantly increased in response to rhIL-3, rhIL-3 plus rhIL-6, and rhIL-3 plus rhGM-CSF, but not in response to rhGM-CSF alone. Furthermore, both meg progenitors were investigated for their response to rh transfer growth factor (TGF)-beta 1, tested at concentrations from 0.01 to 10 ng/ml. rhTGF-beta 1 was able to inhibit CFU-meg and BFU-meg in a dose-response manner normal, whereas ET CFU-meg appeared less sensitive to the lower doses investigated (p less than 0.05) and ET BFU-meg were slightly reduced in number only at the higher concentrations of rhTGF-beta 1 (p less than 0.01). Our data suggest that the increased thrombopoiesis in ET may depend on an increased sensitivity of meg progenitors to some of the physiological growth factors and to a disrupted sensitivity to at least one negative regulator of megakaryocytopoiesis. Since these abnormalities involve both meg progenitors, this can be considered a demonstration that the neoplastic event hits the most primitive hemopoietic progenitors.