Arachidonic acid oxygenation products were for many years held to be syn onymous with prostaglandins, but more recently they have been shown to include thromboxane A2 and a multitude of lipoxygenase pathway products, most notably the leukotrienes. In their review of inflammatory mediators in the first volume of this series, Larsen & Henson (1) provided a broad per spective on how prostaglandins and leukotrienes play a central, often syner gistic role with other mediators, both in the expression of the efferent limb of immune-based inflammatory responses and in acute inflammatory responses where specific immunity does not have a primary role. Prostaglandins and leukotrienes are produced by most, if not all cells, par ticipating in the afferent and efferent limbs of the immune system during host defense and inflammatory responses. Their formation depends on the deacy lation of arachidonic acid from cellular phospholipids catalyzed by the activity