Biosynthesis of 5,15-dihydroxyeicosatetraenoic acid (5,15-diHETE) in leukocytes involves consecutive oxygenation of arachidonic acid by 5-LOX and 15-LOX in either order. Here, we analyzed the contribution of COX-2 to the biosynthesis of 5,15-diHETE and 5,11-diHETE in isolated human leukocytes activated with LPS and calcium ionophore A23187. Transformation of arachidonic acid was initiated by 5-LOX providing 5S-HETE as a substrate for COX-2 forming 5S,15S-diHETE, 5S,15R-diHETE, and 5S,11R-diHETE as shown by LC-MS and chiral phase HPLC analyses. The levels of 5,15-diHETE were 0.45 +/- 0.2 ng/106 cells (mean +/- SEM, n=6), reaching about half the level of LTB4 (1.3 +/- 0.5 ng/106 cells, n=6). The COX-2 specific inhibitor NS-398 reduced the levels of 5,15-diHETE to below 0.02 ng/106 cells in four out of six samples. Similar reduction was achieved by MK-886, an inhibitor of 5-LOX activating protein. Aspirin-treatment of the activated cells allowed formation of 5,15-diHETE (0.1 +/- 0.05 ng/106 cells, n=6) but, as expected, abolished formation of 5,11-diHETE. The mixture of activated cells also produced 5S,12S-diHETE with the unusual 6E,8Z,10E double bond configuration, implicating biosynthesis by 5-LOX and 12-LOX activity rather than by hydrolysis of the leukotriene A4-epoxide. Exogenous octadeuterated 5S-HETE and 15S-HETE were converted to 5,15-diHETE, implicating that multiple oxygenation pathways of arachidonic acid occur in activated leukocytes. The contribution of COX-2 to the biosynthesis of dihydroxylated derivatives of arachidonic acid provides evidence for functional coupling with 5-LOX in activated human leukocytes.