Lipoxygenases (LOs) are non-heme iron containing enzymes which catalyze the hydroperoxidation of polyunsaturated fatty acids. Mammalian LOs have great physiological and pathological importance as they play key roles in the biosynthesis of leukotrienes and Lipoxins. Electron paramagnetic resonance (EPR) studies have shown that ferric active sites of soybean lipoxygenase-1 (SLO-1) and human 5-lipoxygenase (5-HLO) exhibit axial EPR patterns which convert to rhombic spectra upon addition of hydroperoxide product (13-HPOD). In this report, we extend EPR studies to rabbit (15-RLO) and human (15-HLO) mammalian 15-LOs. The spectra of 15-RLO and 15-HLO have rhombic high-spin ferric EPR signals which convert to axial upon interaction with product, opposite to the behavior reported for SLO-1 and 5-HLO, which is indicative of a significant structural difference between the ferric sites of these mammalian 15-LOs and those of SLO-1 and 5-HLO. This appears to relate to the substitution of an asparagine ligand in the SLO-1 and 5-HLO active sites for a histidine ligand in the rabbit and human 15-LOs. A ligand field model of the zero-field splitting is presented which accounts for this opposite EPR spectral behavior in terms of ligand strength differences associated with the Asn (weak) --> His (strong) ligand substitution.