Macrophage migration inhibitory factor (MIF) is a cytokine that plays a critical role in innate and adaptive immunity. It is an upstream activator of monocytes/macrophages and is involved in the pathogenesis of septic shock, arthritis, and other inflammatory conditions. MIF is encoded by a unique but highly conserved gene and has a novel protein fold and structural superfamily. Although the signal transduction pathways activated by MIF have been studied, the nature of its membrane receptor has remained unknown. This study identifies CD74, a type II transmembrane protein, as a high-affinity binding protein for MIF. MIF binds to the extracellular domain of CD74, and CD74 is required for MIF-induced activation of the extracellular signal-regulated kinase-1/2 (ERK-1/2) MAP kinase cascade, cell proliferation, and PGE2 production. A recombinant, soluble form of CD74 binds MIF with a dissociation constant of approximately 9×10⁻⁹ M, as determined by surface plasmon resonance. Soluble CD74 inhibits MIF-mediated ERK activation in defined cell systems. These findings provide a molecular basis for MIF's interaction with target cells and identify CD74 as a natural ligand for MIF. The study also demonstrates that CD74 is required for MIF-induced ERK-1/2 phosphorylation, PGE2 production, and cell proliferation. These results highlight the importance of CD74 in MIF signaling and its role in immune cell activation.Macrophage migration inhibitory factor (MIF) is a cytokine that plays a critical role in innate and adaptive immunity. It is an upstream activator of monocytes/macrophages and is involved in the pathogenesis of septic shock, arthritis, and other inflammatory conditions. MIF is encoded by a unique but highly conserved gene and has a novel protein fold and structural superfamily. Although the signal transduction pathways activated by MIF have been studied, the nature of its membrane receptor has remained unknown. This study identifies CD74, a type II transmembrane protein, as a high-affinity binding protein for MIF. MIF binds to the extracellular domain of CD74, and CD74 is required for MIF-induced activation of the extracellular signal-regulated kinase-1/2 (ERK-1/2) MAP kinase cascade, cell proliferation, and PGE2 production. A recombinant, soluble form of CD74 binds MIF with a dissociation constant of approximately 9×10⁻⁹ M, as determined by surface plasmon resonance. Soluble CD74 inhibits MIF-mediated ERK activation in defined cell systems. These findings provide a molecular basis for MIF's interaction with target cells and identify CD74 as a natural ligand for MIF. The study also demonstrates that CD74 is required for MIF-induced ERK-1/2 phosphorylation, PGE2 production, and cell proliferation. These results highlight the importance of CD74 in MIF signaling and its role in immune cell activation.