Human CD116 is the alpha subunit of granulocytemacrophage colony stimulating factor receptor (GMCSFR, also called colony stimulating factor 2 receptor, alpha) that binds GM-CSF with low affinity. The molecule was termed CD116 at the 5th International Workshop on Leukocyte Differentiation Antigens (IWLDA, Boston, USA, 1993). The beta subunit (CD131), which is also shared with the IL3 and IL5 receptors, has no detectable binding affinity for GM-CSF on it’s own but is necessary for high affinity binding when associated with the alpha subunit and plays a fundamental role in signal transduction. Monoclonal antibodies (MoAbs) against CD116 (extracellular domain) are used for phenotyping various cell populations possibly contributing to the diagnosis and therapy of acute myeloid leukemia (AML). In AML, GM-CSFR is detectable in 60-70% of cases and particularly in M4 and M5 FAB subvarieties. The number of receptors expressed by AML cells is sometimes significantly higher than that in normal hematopoietic cells, suggesting the possibility of using this marker as a useful tool for the monitoring of minimal residual disease. Cellular CD116 expression was documented in > 50% M0 AML. Since the activity of GM-CSF on hemopoietic cells depends upon its binding to specific cell surface receptors, we have previously hypothesised that the clinical use of GM-CSF in AML patients could be optimized by a dynamic analysis of the number and the affinity status of GM-CSFR in leukemic blasts and normal hemopoietic cells .