This stabilized and partially purified porcine leukocyte enzyme has a specific activity of approximately 50 units/mg protein. The enzyme is suspended in 20 mM Tris pH 7.2 containing 0.5 mM EDTA, PMSF, DTT, and 40% glycerol. Stable at -70°C. Labile to freezing and thawing. One unit is the amount of enzyme that catalyzes the conversion of one nanomole of arachidonic acid to 12-HPETE during a minute assay at room temperature.