Arachidonate 12-lipoxygenase purified from porcine leukocytes shows 14R-oxygenase and 14, 15-leukotriene A synthase activities with 15-hydroperoxy-arachidonic acid as substrate. The enzyme transformed 5, 15-dihydroperoxy-arachidonic acid to several compounds with a conjugated tetraene. A major product was identified as 5S, 14R, 15S-trihydroperoxy-6, 10, 12-trans-8-cis-eicosatetraenoic acid, which was reduced to 5S, 14R, 15S-8-cis-lipoxin B. A requirement of molecular oxygen and the results of H2(18)O experiments suggested that formation of the latter compound was attributed mostly to the 14R-oxygenase activity of the enzyme. There were several other minor products identified as lipoxin A and B isomers. They were produced presumably by hydrolysis of 14, 15-epoxy compound formed by the leukotriene A synthase activity of 12-lipoxygenase.