Granulocyte-macrophage colony-stimulating factor (GMCSF) has a central role in proliferation and differentiation of hematopoetic cells. Furthermore, it influences the proliferation and migration of endothelial cells. GMCSF elicits these functions by activating a receptor consisting of a ligand-specific α-chain and a β-chain, which is common for GMCSF, interleukin-3 (IL-3), and IL-5. It is known that various signaling molecules such as Janus kinase 2 or transcription factors of the signal transducer and activator of transcription (STAT) family bind to the common β-chain and initiate signaling cascades. However, α-chain—specific signal transduction adapters have to be postulated given that IL-3, IL-5, and GMCSF induce partly distinct biologic responses. Using a yeast 2-hybrid system, we identified the α-chain of the GMCSF receptor (GMRα) as putative interaction partner of IκB kinase β, one of the central signaling kinases activating the transcription factor nuclear factor—κB (NF-κB). Using endogenous protein levels of endothelial cell extracts, we could verify the interaction by coimmunoprecipitation experiments. Fluorescence resonance energy transfer (FRET) microscopy confirmed the direct interaction of CFP-IKKβ and YFPGMRα in living cells. Functional studies demonstrated GMCSF-dependent activation of IκB kinase activity in endothelial cells, degradation of IκB, and activation of NF-κB. Further biologic studies using GMCSF-dependent TF-1 cells indicated that GMCSF-triggered activation of NF-κB is important for cell survival and proliferation. (Blood. 2003;102:192-199)