Effect of ε-fructoselysine on the activity of disaccharidase in the small intestine of male albino rats in vivo and in vitro systems was investigated. ε-Fructoselysine (Amadori compound, 1-deoxy-l-(-N-L-lysino)-D-fructose) was prepared from. browned casein which was formed 10g of milk casein and 1.6g of glucose under relatively mild conditions (37℃±0.5, R.H. 70±3%), and its purity was confirmed with the synthetic standard (Rf value, 0.23). Disaccharidase activities in the rats fed 5% ε-fructoselysine diet were proved to be significantly reduced. Using a model system (ε-iructoselysine), the in vitro study on the mode of inhibitory effect on maltase activity revealed that the comopund showed a competitive inhibitory effect (Ki=3.6mo1). When browned casein (condition mentioned above) was ingested by growing rats, ε-fructo selysine which was found after 3 hr, was remained from the intestinal TCA-soluble fraction, but L-lysine disappeared after 7hr from this result, ε-fructoselysine which remained as an absorption-delayed material in the small intestinal lumen is responsible for its inhibitory effect.