Eleven men received a fish oil (2.4 g n-3 fatty acids), fish oil plus vitamin E (300 IU) or no supplement for 3 weeks. Dietary fish oil increased the maximum amount of conjugated dienes formed during copper-catalysed oxidation of LDL in vitro by 18-20%. Fish oil also tended to decrease the lagtime before onset of oxidation, but this effect was counteracted by vitamin E. We conclude that fish oil supplementation increases the oxidizability of LDL in vitro, and vitamin E increases its oxidation resistance.