The omega-3 fatty acid eicosapentaenoic acid (EPA) reduces oxidation of ApoB-containing particles in vitro and in patients with hypertriglyceridemia. EPA may produce these effects through a potent antioxidant mechanism, which may facilitate LDL clearance and slow plaque progression. We hypothesize that EPA antioxidant effects may extend to ApoA-containing particles like HDL, potentially preserving certain atheroprotective functions. HDL was isolated from human plasma and incubated at 37 degrees C in the absence (vehicle) or presence of EPA and/or DHA; 5.0 or 10.0 mu M each. Samples were then subjected to copper-induced oxidation (10 mu M). HDL oxidation was inhibited similarly by EPA and DHA up to 1 h. EPA (10 mu M) maintained significant HDL oxidation inhibition of 89% (0.622 +/- 0.066 IN MDA; p < .001) at 4 h, with continued inhibition of 64% at 14 h, vs. vehicle (5.65 +/- 0.06 to 2.01 +/- 0.10 mu M MDA; p < .001). Conversely, DHA (10 mu M) antioxidant benefit was lost by 4 h. At a lower concentration (5 mu M), EPA antioxidant activity remained at 81% (5.53 +/- 0.15 to 1.03 +/- 0.10 mu M MDA; p <.001) at 6 h, while DHA lost all antioxidant activity by 4 h. The antioxidant activity of EPA was preserved when combined with an equimolar concentration of DHA (5 mu M each). EPA pretreatment prevented HDL oxidation in a dose dependent manner that was preserved over time. These results suggest unique lipophilic and electron stabilization properties for EPA as compared to DHA with respect to inhibition of HDL oxidation. These antioxidant effects of EPA may enhance certain atheroprotective functions for HDL (C) 2018 The Authors. Published by Elsevier Inc.