Clonogenic cell culture assay was used to evaluate the effect of mast cell growth factor (MGF) on peripheral blood granulocyte-macrophage (GM) progenitors in 26 patients with myeloproliferative disorders (MPDs). MGF alone had a statistically significant stimulatory effect on GM colony formation, as also did interleukin-3 (IL-3) and GM colony-stimulating factor (GM-CSF), although the progenitors could form colonies spontaneously as well. When MGF was combined with either IL-3 or GM-CSF the effect was additive and was as great as that achieved with a mixture of IL-3, GM-CSF, G-CSF and IL-6. The highest colony-forming capacity of all was seen when MGF was added to the above mixture. Within the subgroups of MPDs, the stimulatory effect of MGF was significant in polycythemia vera (PV), essential thrombocythosis (ET) and chronic myelogenous leukemia (CML). MGF was the most potent single factor in PV, while GM-CSF was most effective in idiopathic myelofibrosis and both IL-3 and GM-CSF in CML. The fact that the ability of MGF to induce colony growth varied between the subgroups of MPDs may mean that the target progenitors in these diseases are biologically different. In conclusion, MGF, either alone or with others, was a potent growth factor for GM progenitors in MPDs.