Natto is a Japanese traditional food made from whole soybeans by the fermentation of Bacillus natto. The purpose of this study was to investigate the effect of Natto mucilage on lipid metabolism in rats. The levels of total cholesterol, LDL-cholesterol and triglyceride in serum of rats fed Natto mucilage diets were lower than those in control rats, but serum HDL-cholesterol and phospholipid were higher than those in control rats. The results indicate that Natto mucilage may have more beneficial roles in lipid metablism because it decreased serum cholesterol and increased bilirubin excretion.