Since pyridoxamine, which traps intermediates in the Maillard reaction and lipid peroxidation reaction, significantly inhibits the development of retinopathy and neuropathy in the streptozotocin-induced diabetic rat, treatment with advanced glycation end product inhibitors and antioxidants may be a potential strategy for the prevention of clinical diabetic complications. However, the paradoxical effect of green tea has been reported; although plasma hydroperoxide levels were ameliorated, the level of N epsilon-(carboxyethyl)lysine (CML) in tendon and plasma was increased by the oral administration of green tea to diabetic rats. In the present study, we measured the effect of natural compounds on CML formation by enzyme-linked immunosorbent assay. A significant amount of CML was observed when bovine serum albumin was incubated with ribose for 7 days. Under the same conditions, natural compounds, such as desgalactotigonin, showed inhibitory effects, whereas quercetin and acteoside enhanced CML formation, indicating that natural compounds contain both inhibitors and enhancers for CML formation.