The complete amino acid sequence of arachidonate 12-lipoxygenase (EC 1.13.11.31) of porcine leukocytes was deduced by cloning and sequence analysis of DNA complementary to its mRNA. The sequence was confirmed by automated Edman degradation of the N-terminal regions of the native enzyme and its proteolytic fragments. The cDNA had an open reading frame encoding 662 amino acid residues with a calculated molecular weight of 74,911. Amino acid residues 533-545, Cys-(Xaa)3-Cys-(Xaa)3-His-(Xaa)3-His, showed significant homology to the short cysteine- or histidine-containing sequences proposed as the metal-binding domains of transcription factors and various metal-containing proteins [Berg, J. M. (1986) Science 232, 485-487]. The amino acid sequence of 12-lipoxygenase exhibited 86% identity with human reticulocyte 15-lipoxygenase and showed 41% identity with human leukocyte 5-lipoxygenase. The 12-lipoxygenase cDNA recognized a 3.4-kilobase mRNA species in various porcine cell types, with the largest amount in leukocytes, followed by pituitary, lung, jejunum, and spleen.