Originally described as a T lymphocyte-derived factor that inhibited the random migration of macrophages, the protein known as macrophage migration inhibitory factor (MIF) was an enigmatic cytokine for almost 3 decades. In recent years, the discovery of MIF as a product of the anterior pituitary gland and the cloning and expression of bioactive, recombinant MIF protein have led to the definition of its critical biological role in vivo. MIF has the unique property of being released from macrophages and T lymphocytes that have been stimulated by glucocorticoids. Once released, MIF overcomes the inhibitory effects of glucocorticoids on TNF alpha, IL-1 beta, IL-6, and IL-8 production by LPS-stimulated monocytes in vitro and suppresses the protective effects of steroids against lethal endotoxemia in vivo. MIF also antagonizes glucocorticoid inhibition of T-cell proliferation in vitro by restoring IL-2 and IFN-gamma production. This observation has identified a pivotal role for MIF within the immune system and fills an important gap in our understanding of the control of inflammatory and immune responses. Glucocorticoids have long been considered to be an integral component of the stress response to infection or tissue invasion and serve to modulate inflammatory and immune responses. MIF is the first mediator to be identified that can counter-regulate the inhibitory effects of glucocorticoids and thus plays a critical role in the host control of inflammation and immunity.