ABSTRACT: Native and heated soy protein isolate was hydrolyzed with 3 purified (pepsin, papain, and chymotrypsin) and 3 crude (Alcalase®, ProtamexTM, and FlavourzymeTM) proteases. The hydrolysates were incubated (37 °C, 1 h) with a liposome-oxidizing system (50 μM FeCl3/0.1 mM ascorbate, pH 7.0) to test antioxidant activities by determining the concentrations of TBARS. Degree of hydrolysis of SPI hydrolysates ranged from 1.7 to 20.6%. Both hydrolyzed and nonhydrolyzed SPI decreased TBARS (by 28 to 65%), except for papain-hydrolyzed samples. Samples of chymotrypsin- and Flavourzyme-hydrolyzed (0.5 h) preheated SPI had the greatest inhibitory effect on lipid oxidation.