The protein solubilized during the production of soy concentrates was recovered by ultrafiltration and subjected to enzyme hydrolysis with a commercial protease. The raw proteins separated by ultrafiltration and the hydrolyzates of high molecular weight fractions were assayed for antioxidant activity. The fraction with molecular weight < 10 kDa presented the highest antioxidant activity using all the methods considered. Hydrolyzates showed higher reducing power, higher antioxidant potency in emulsion and enhanced radical scavenging capacity than the original fractions, whereas their ability for inhibiting the beta-carotene bleaching was comparable to those of synthetic antioxidants. Hydrolyzates from the fraction with molecular weight between 30 and 50 kDa showed the highest hydroxyl radical scavenging capacity, and the highest Trolox Equivalent Antioxidant Capacity (TEAC) (up to 6.6 mM Trolox) was recorded for hydrolyzates from the fraction with molecular weight higher than 50 kDa. (c) 2005 Elsevier Ltd. All rights reserved.