Aged garlic extract (AGE) has been shown to have antioxidant activity. The organosulfur compounds, S-allyl-L-cysteine and S-allylmercapto-L-cysteine, are responsible, at least in part, for the antioxidant activity of AGE. To identify major active components, we fractionated AGE, using hydrogen peroxide scavenging activity as an antioxidative index. Strong activity in the amino acid fraction was found and the major active compound was identified as N alpha-(1-deoxy-D-fructos-1-yl)-L-arginine (Fru-Arg). Antioxidant activity of Fru-Arg was comparable to that of ascorbic acid, scavenging hydrogen peroxide completely at 50 micromol/L and 37% at 10 micromol/L. Quantitative analysis using the established HPLC system revealed that AGE contained 2.1-2.4 mmol/L of Fru-Arg, but none was detected in either raw or heated garlic juice. Furthermore, it was shown that a minimum of 4 mo aging incubation was required for Fru-Arg to be generated. These findings indicate that the aging process is critical for the production of the antioxidant compound, Fru-Arg. These results may explain some of the variation in benefits among different commercially available garlic preparations.