The biosynthesis of leukotrienes (LT) C 4 and B 4 is followed by an export of these mediators into the extracellular space. This transport was characterized using plasma membrane vesicles prepared from mastocytoma cells and identified as an ATP-dependent primary active process. The apparent K m -values were 110 nM for LTC 4 and 48 μM for ATP. The transport rate was highest for LTC 4 , whereas LTD 4 , LTE 4 , and N-acetyl-LTE 4 were transported with relative rates of 31, 12 and 8%, respectively, at a concentation of 10nM. LTB 4 transport was also dependent on ATP, LTC 4 transport with inhibited by LTD 4 receptor antagonists (IC 1 0 = 1.0 μM for MK-571 and 1.3 μM for LY245769) and by the inhibitor of leukotriene biosynthesis MK-886 (IC 1 0 = 1.8 μM). The ATP-dependent export carrier for leukotrienes in leukotriene-synthesizing cells represents a novel member of the family of ATP-dependent exit pumps.