A flow cytometric method to quantify the Granulocyte-macrophage colony-stimulating factor receptor (GM-CSFr) on human cells is described. The number of GM-CSFr binding sites on human neutrophils was assessed by using different bead standards. Results were compared with those from conventional receptor quantification, which was performed by using the radioligand binding assay. A high degree of correlation was found between the two methods, although quantitative evaluation of GM-CSFr expression on neutrophils performed by flow cytometry revealed a somewhat higher number of receptor molecules per cell than with that determined by Scatchard analysis. By the flow quantitative approach, we measured the GM-CSFr on mobilized CD34-positive cells and obtained results similar to those of previously published data. Our data suggest that flow cytometric analysis is a simple and reproducible method to detect and quantify the presence of GM-CSFr per cell, thus allowing the study of receptor expression on different populations selected by gating on the basis of the scatter parameters and surface markers. This assay offers the possibility to quantify the presence of GM-CSFr on different subsets of normal and pathological cells even if samples are too small (such as CD34-positive progenitor cells) for measurement with the radioligand binding assay.