LTB4 is synthesized from arachidonic acid by the combined action of 5-LO and LTA4 hydrolase. LTB4 has long been recognized as a potent mediator of inflammation. It stimulates a number of leukocyte functions, including aggregation, stimulation of ion fluxes, enhancement of lysosomal enzyme release, superoxide anion production, chemotaxis, and chemokinesis. In subnanomolar ranges ,(3.9 x 10-10 M), LTB4 causes chemotaxis and chemokinesis in human PMNLs. At higher concentrations, (1.0 x 10-7 M), LTB4 leads to neutrophil aggregation and degranulation as well as superoxide anion production. Plasma levels of LTB4 increase from 100 ng/ml following leukocyte stimulation. LTB4 is metabolized in leukocytes and hepatocytes to less active 20-hydroxy and 20-carboxy LTB4 and is not excreted in the urine.