Hyperresponsiveness of Mpl^+/+ c-Myb^Plt4/Plt4 mutant megakaryocyte progenitor cells, but not granulocyte and/or macrophage progenitors, to GM-CSF. The unbroken lines depict similar GM-CSF dose response curves for granulocyte-macrophage colony formation from 2 Mpl^+/+ c-Myb^Plt4/Plt4 and a control Mpl^+/+ c-Myb^+/+ bone marrow. In contrast, for megakaryocyte colony formation, Mpl^+/+ c-Myb^Plt4/Plt4 marrow exhibited hyperresponsiveness (broken line), with maximal stimulation at doses similar to that for granulocyte-macrophage colony formation. No megakaryocyte colonies were evident from Mpl^+/+ c-Myb^+/+ marrow at the doses shown; 1:1 represents a GM-CSF concentration of 10 ng/mL.