The cytokine macrophage migration inhibitory factor (MIF) is a major protein constituent of the anterior pituitary gland released into the bloodstream during endotoxaemia. For many years, MIF had been thought to be a T cell product associated with delayed-type hypersensitivity reactions. The identification of MIF as a pituitary 'stress' hormone provides an important link in the regulation of systemic inflammatory responses by the central nervous system.