Macrophage migration inhibitory factor(MIF) was discovered as a soluble factor associated with the mechanism of delayed-type hypersensitivity. At present, MIF is accepted as a multifunctional cytokine involved in a broad-spectrum pathological events relevant to the immune system and tumorigenesis. Following the cloning of human MIF cDNA, novel biological functions of MIF have been demonstrated. For example, MIF is released from the anterior pituitary gland in endotoxin stimuli, and as a proinflammatory cytokine and glucocorticoid-induced immunomodulator produced by a variety of inflammatory stimuli. From these findings, it is conceivable that MIF would be a novel indicator, suggesting systemic inflammatory conditions, such as septicemia. Recently, it was found that Toll-like receptor 4 could be regulated by MIF, suggesting its involvement in the innate immunity.