The control of blood cell production is modulated directly or indirectly by more than ten different hematopoietic growth factors (Metcalf 1984). Many names have been assigned to each of the hematopoietic growth factors: although a specific set of names is used throughout this article, the alternative names can be found in other reviews (Nicola and Vadas 1984; Metcalf 1985; Arai et al. 1986; Morstyn and Burgess 1988). Six of these growth factors appear to be capable of stimulating the proliferation of hematopoietic progenitor cells — multi-colony-stimulating factor (multi-CSF, also called interleukin-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), macrophage colony-stimulating factor (M-CSF, also called CSF-1), interleukin-5 (also called eosinophil differentiation factor), and erythropoietin. The existence of these different growth factors had been apparent for almost 20 years, but in the last 5 years all of these molecules have been purified, cloned, and expressed. We know that there is no structural relationship between the regulators, but there is a considerable overlap in their biological activities. Multi-CSF, GM-CSF, and interleukin-5 all stimulate the production of eosinophils (Metcalf et al. 1986; Lopez et al. 1986); multi-CSF (Ihle et al. 1982; Burgess et al. 1980), GM-CSF (Burgess et al. 1977 a, 1986), and M-CSF (Stanley and Guilbert 1981) stimulate the production of macrophages; and multi-CSF, GM-CSF, and G-CSF lead to the production and stimulation of neutrophils. The redundancy in these functional activities occurs at the level of cell surface receptor interactions, so it is necessary to consider some of the target cell receptor biochemistry for several of the hematopoietic growth factors as well as considering the properties of a single molecule such as GM-CSF.