YTOKINES ARE actively involved in hematopoiesis C by regulating proliferation, differentiation, and cellular functions of various lineages of hematopoietic cells.’Two fundamental characteristics for cytokines have been noticed, ie, each cytokine exhibits pleiotropic functions on different target cells and a subset of cytokines often shows similar functions on the same target cell.’The structures of the high-affinity receptors for these cytokines shown by molecular cloning provide a basis for the functional redundan~ y.~,~ Unlike growth factor receptors with an intrinsic tyrosine kinase, the high-affinity receptors for hematopoietic cytokines consist of multiple subunits and the receptors for a subset of cytokines with similar functions share the common component essential for their signal transduction. This review will discuss recent findings concerning the structure, expression, and function of the receptors for a subset of hematopoietic cytokines, granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (1L-3), and IL-5, which exhibit similar biologic functions in their common target cells.