ABSTRACT Polyphenols and particularly flavonoids are well known in vitro antioxidants. Their consumption in foods has been shown to decrease the risk of heart disease in epidemiological studies. We examined two commonly consumed nonalcoholic juices (grape juice and orange juice) for their ability to act as in vitro plasma antioxidants, enrich lower-density lipoproteins after plasma spiking, and protect these lipoproteins from oxidation after supplementation to healthy subjects. We found that grape juice, but not orange juice, possesses all of these antioxidant properties and is an excellent nonalcoholic alternative to red wine. Grape juice is a powerful in vivo antioxidant, and this property, in combination with its platelet aggregation inhibition ability, can potentially reduce the risk of heart disease.