Aerobic metabolism in biological systems produces reactive oxygen species, and defense against such prooxidants requires antioxidant activity, e.g., predominantly vitamins C and E in serum. Recently, flavonoids (polyphenols occurring widely in plants) have been investigated in vitro for their antioxidant activity; whether they are absorbed after ingestion is not clear. Using a chemiluminescent assay of serum antioxidant capacity (SAOC), we have studied the effects in normal individuals of ingesting red wine, white wine, and high doses of vitamin C. In nine subjects who ingested 300 mL of red wine, the mean SAOC was increased by 18% after 1 h and by 11% at 2 h. The same amount of white wine produced 4% and 7% increases, respectively. The ingestion of 1000 mg (5.7 mmol) of ascorbic acid by four subjects increased their mean SAOC by 22% at 1 h and by 29% at 2 h. An in vitro comparison of red wine, white wine, and various fruit juices showed the high antioxidant capacity of red wine in addition to its ability to increase the antioxidant capacity of serum in vivo. The antioxidant effects of various flavonoids and other polyphenols were also studied.