Ingestion of soy protein isolate (SPI) lowers the serum total cholesterol concentration in humans as well as experimental animals. Thus, SPI ingestion may reduce the risk of atherosclerosis. The antiatherogenic effect of SPI was reviewed based on our animal experiments. SPI was beneficial for reducing atherosclerosis in both hypercholesteromic rats and apolipoprotein E-deficient mice. This antiatherogenic effect of SPI was attributed to reduction of serum cholesterol and improvement of arterial lipid metabolism. The active component in SPI appeared to be in the protein part.