Recent studies have led to the discovery of a mediator that appears to act as an endogenous hormone to counterregulate glucocorticoid action within the immune system. Isolated as a product of anterior pituitary cells, the structure of this protein was found to be that of macrophage migration inhibitory factor (MIF)--one of the first cytokines to be identified and described originally as a T lymphocyte factor that inhibited the random migration of macrophages. Macrophages and T cells release MIF in response to glucocorticoids, as well as upon activation by various proinflammatory stimuli. Once secreted MIF "overrides" the immunosuppressive effects of steroids on macrophage and T-cell cytokine production. MIF appears to fill an important gap in our understanding of how the host initiates and controls the immune response. Because glucocorticoids are an integral part of the host's metabolic "stress" response to infection or tissue invasion, the role of MIF is to act at an inflammatory site or lymph node to counterbalance the inhibitory effects of steroids on the immune response. Anti-MIF therapeutic strategies are under development and may prove to be a means to increase the immunosuppressive and anti-inflammatory properties of endogenously released glucocorticoids, thereby reducing the requirement for steroid therapy in a variety of autoimmune and inflammatory conditions.