Natto, one of Japanese traditional food is made from steamed whole soybeans fermented with Bacillus natto. In this study, the effects of Natto mucilage- feeding on griwth, organ weight and serum nitrogen compounds of rats were investigated. Male rats in Sprague-Dawley strain were fed on basal diets supplemented with aqueous Natto mucilage of several percentages for 10 weeks. Natto mucilage had no significant effects on the body and organ weights. The levels of GOT, GPT, LDH. and total bilirubin in serum of rats fed Natto mucilage diets were significantly lower than those in the control group ' The levels of total protein and albumin were higher than those in the control group. The level of calcium in serum indicates a growing tendency. but creatinine, uric acid, and BUN had declining. And the most component of free am1no acids in Natto mucilage were glutamin acid.