The iLite® GM-CSF Assay Ready Cells can be used for the quantification GM-CSF activity, GM-CSF inhibitor activity, and for determination of neutralizing antibodies against such either in buffer systems or human serum. Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a cytokine which stimulates the production of granulocytes and monocytes from bone marrow precursors (1). As such, it serves as an important key in both humoral and cell mediated immunity. Recombinant GM-CSF has several therapeutic uses; in order to accelerate leukocyte recovery after bone marrow transplantation, to replenish leukocytes after chemotherapy and for treatment of fungal infections. The immunostimulatory effects of GM-CSF have also been used in oncolytic viruses modified to include genes coding for GM-CSF, thus enhancing recruitment of the immune response to tumor cells. In addition, the discovery of a pro-inflammatory role of GM-CSF in autoimmune disease has led to development of several GM-CSF inhibitor drugs.