The cytokine macrophage migration inhibitory factor (MIF) has emerged recently as an important mediator of inflammation and innate immunity. MIF is rapidly released by macrophages after stimulation with microbial products and pro-inflammatory cytokines and, in turn, stimulates the production of pro-inflammatory mediators by immune cells. Immunoneutralization of MIF or deletion of the Mif gene was shown to protect animals from lethal endotoxemia, staphylococcal toxic shock and septic shock in experimental models of bacterial peritonitis. To investigate the function of MIF in innate immunity, we studied the response of macrophages expressing reduced levels of MIF to microbial products. These cells were generated by transduction of an antisense MIF adenovirus or by stable transfection with an antisense MIF plasmid or were obtained from MIFknockout mice. MIF-deficient macrophages were shown to be hyporesponsive to stimulation with LPS and Gram-negative bacteria. The defect was associated with a down-regulation of Toll-like receptor 4 (TLR4), the signal transducing molecule of the LPS receptor complex. Immunoneutralization of extracellular MIF decreased TLR4 expression and responses of macrophages to LPS, indicating that MIF may exert autocrine effects. These findings identify an important role for MIF in innate immunity and provide a rationale for the development of anti-MIF strategy for the treatment of patients with Gram-negative septic shock.