Arachidonate 12-lipoxygenase was purified to near homogeneity from the cytosol fraction of porcine leukocytes by ammonium sulfate fractionation, DEAE-cellulose chromatography, and immunoaffinity chromatography using a monoclonal antibody against the enzyme. The purified enzyme was unstable (half-life of about 24 h at 4 degrees C) but was markedly protected from the inactivation by storage in the presence of ferrous ion or in the absence of air. The lag phase which was observed before the start of the enzyme reaction was abolished by the presence of 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid. An apparent substrate inhibition was observed with arachidonic acid and other active substrates; however, the substrate concentration curve was normalized by the presence of 0.03% Tween 20. Arachidonic acid was transformed to the omega-9 oxygenation product 12-hydroperoxy-5Z,8Z,10Z,14Z-eicosatetraenoic acid. C-12 oxygenation also occurred with 5-hydroxy- and 5-hydroperoxyeicosatetraenoic acids; the respective maximal velocities were 60 and 150% of the rate with arachidonic acid. Octadecaenoic acids were also good substrates. gamma-Linolenic acid was oxygenated in the omega-9 position (C-10), while linoleic and alpha-linolenic acids were subject to omega-6 oxygenation (C-13). A far more complex reaction was observed using 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid as substrate. Reaction occurred at 70% of the rate with arachidonic acid. The dihydroperoxy and dihydroxy products were identified by their UV absorption spectra, high performance liquid chromatography, and gas chromatography-mass spectrometry. Among these products, (8S,15S)-dihydroperoxy-5Z,9E,11Z,13E-eicos atetraenoic acid and (14R,15S)-erythro-dihydroperoxy-5Z,8Z,10E, 12E-eicosatetraenoic acid were produced in larger amounts than the (8R)- and (14S,15S)-threo isomers, respectively; these products were attributed to 8- and 14-oxygenation of the 15-hydroperoxy acid. Furthermore, formation of 14,15-leukotriene A4 was inferred from the characteristic pattern of its hydrolysis products comprised of equal amounts of (8R,15S)- and (8S,15S)-dihydroxy-5Z,9E,11E,13E-eicosatetraenoi c acids together with smaller amounts of (14R,15S)-erythro- and (14S,15S)-threo-dihydroxy-5Z,8Z,10E,12E-eicosate traenoic acids. Thus, both lipoxygenase and leukotriene synthase activities were demonstrated with the homogeneous preparation of porcine leukocyte 12-lipoxygenase.