Human tracheal epithelial cells contain an arachidonate 15-lipoxygenase, while the same cells from animals (including bovine, ovine, canine, porcine) cells express a 12-lipoxygenase. The epithelial 12-lipoxygenase is antigenically related to the leukocyte 12-lipoxygenase but is biochemically distinct from platelet and leukocyte forms of the enzyme, in that it is more efficient at metabolizing a wider array of fatty acid substrates. We have suggested that this lipoxygenase heterogeneity may provide a basis for different functional roles for the enzyme in different cell types. In addition, animal epithelial 12-lipoxygenase and human epithelial 15-lipoxygenase are antigenically related and have similar but distinct distributions in the lung. Our findings might suggest that the species diversity for epithelial lipoxygenases represents molecular divergence within a family of closely related genes with perhaps closely related functions.