Lipoxin A4 (LXA4 ) is a biologically active lipoxygenase interaction product derived from arachidonic acid. Arachidonic acid is first oxygenated by 15-lipoxygenase to form 15-HETE which is then converted by 5-lipoxygenase and epoxide hydrase to generate LXA4 . LXA4 regulates leukocyte traffic, inhibiting neutrophils and stimulating monocytes. It also inhibits natural killer cell cytotoxicity. Recent findings indicate that LXA4 inhibits leukocyte-dependent inflammation. Determinations of LXA4 levels may provide new understanding of the role of LXA4 in basic cellular reactions and in pathophysiology of inflammation and other disease processes.