Leukotrienes (LTs) are a group of acute inflammatory mediators derived from arachidonic acid in leukocytes. The majority of these metabolites are formed through the 5-lipoxygenase (5-LO) pathway. 14,15-LTC4 is a member of an alternate class of LTs synthesized by a pathway involving the dual actions of 15- and 12-LOs on arachidonic acid via 15-HpETE and 14,15-LTA4 intermediates. 14,15-LTC4 is classified as an eoxin, because it is formed mostly by eosinophils. However, mast cells and nasal polyps can synthesize 14,15-LTC4 as well. Little is known about the physiological actions of 14,15-LTC4. It has weak contractile activity on both guinea pig ileum and pulmonary parenchyma in contrast to the effects of 5-LO-derived LTs. However, in an in vitro permeability assay, 14,15-LTC4 can increase vascular permeability of human endothelial cell monolayers, with similar potency to that of 5-LO-derived LTs resulting in plasma leakage - a hallmark of inflammation.