The goal of this study is to define the role of macrophages in the regulation of erythropoiesis. Macrophages directly influence the growth and development of erythroid progenitors, both in vitro and in vivo. Using a unique model system developed in our laboratory, we have found that macrophage regulation of erythropoiesis is impaired in erythroleukemia. This regulation can be restored and the disease reversed by treatment with normal macrophages. The mechanism by which macrophages regulate normal and leukemic erythropoiesis has been investigated. We have found that there is a regulatory network operative in vivo involving macrophages and erythroid progenitors (CFU-E), and potent positive and negative regulatory mediators. Macrophages produce a factor that greatly reduces in vivo CFU-E levels. This effect of macrophages is reversed by an erythroid stimulatory activity, related to but distinct from erythropoietin, induced by anemic stress and whose activity in vivo is controlled by macrophages. The aims of this study are to isolate and characterize these erythroid regulatory activities; to determine their origins and mechanisms of action; to assess their roles in controlling normal and leukemic erythropoiesis; and to determine the interactions between hematopoietic controls and immune functions known to be involved in spontaneous and induced regression of erythroleukemia, in reversal of the disease. These findings reveal a novel and significant in vivo regulatory circuit for the erythroid compartment, responsive to external stimuli and tightly controlled by positive and negative mediators. The systems we have developed uniquely allow analysis of this regulatory axis, both in vitro and in vivo. Our studies offer the possibility of gaining a greater understanding of erythropoiesis and ultimately a potential for use of this knowledge in the therapy of hematopoietic disorders.