To determine the expression and function of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor alpha chain (GMR alpha) during hematopoiesis and on leukemic cells, monoclonal antibodies were raised by immunizing mice with cells expressing high levels of human GMR alpha. A pool of five antibodies isolated from three different mice was used to characterize GMR alpha. This antibody pool (anti-GMR alpha) immunoprecipitated a protein with the expected molecular weight of GMR alpha from COS cells transiently transfected with the GMR alpha gene. In factor-dependent cells, GMR alpha existed as a phosphoprotein. However, its phosphorylation was not stimulated by the presence of GM- CSF. Anti-GMR alpha inhibited the GM-CSF-dependent growth of cell lines and normal bone marrow cells and inhibited the binding of iodinated GM- CSF to its receptor. Cell surface expression of GMR alpha was examined using anti-GMR alpha and flow cytometry. GMR alpha was readily detectable on both blood monocytes and neutrophils. In adherence- depleted normal bone marrow, two separate populations expressed GMR alpha. The most positive cells were predominantly macrophages, whereas the cells that expressed less GMR alpha were largely myelocytes and metamyelocytes. A small population of lin-CD34+ or CD34+CD38- cells also expressed GMR alpha, but they were not capable of significant growth in colony-forming assays. In contrast, the majority of lin-CD34+ and CD34+CD38- cells were GMR alpha-, yet they produced large numbers of myeloid and erythroid colonies in the same assay. Malignant cells from patients with leukemia were also tested for GMR alpha expression. All of the myeloid leukemias and only rare lymphoid leukemias surveyed tested positive for GMR alpha. These results show that anti-GMR alpha is useful for the functional characterization of the GMR alpha and for the detection of myeloid leukemia and that GMR alpha is expressed on certain lineages throughout hematopoietic development; however, progenitors that express the receptor may have a reduced capacity to proliferate in response to hematopoietic growth factors.