Hemopoiesis, the development of functional blood cells, requires the action of colony-stimulating factors to promote the survival and stimulate the proliferation and differentiation of progenitor cells in the bone marrow [1-3]. Human granulocyte-macrophage colony-stimulating factor (GM-CSF) is a 22,000-dalton glycoprotein produced by T lymphocytes, endothelial cells, and fibroblasts: the gene has been cloned in yeast, Escherichia coli, and monkey Cos cells [4-7]. GM-CSF, together with interleukin-3 and other colony-stimulating factors, promotes the growth and differentiation of colonyforming units for granulocytes (neutrophils and eosinophils), erythrocytes, monocytes, and megakar3"ocytes (fig. 1). It is of special importance in the maturation of myeloid precursors to both granulocytes and monocytes; low concentrations of GM-CSF promote the formation only of macrophages, while high concentrations promote the granulocytic pathway [8].