Glycation, the nonenzymatic adduct formation between sugar dicarbonyls and proteins, is one key molecular basis of diabetic complications due to hyperglycemia. Given the link between glycation and oxidation, we hypothesized that herbal extracts with a high concentration of antioxidant phenolics might possess significant in vitro antiglycation activities as well. The aim of the present study was to address the hypothesis that polyphenol-rich Ilex paraguariensis (IP) extracts are capable of inhibiting advanced glycation end-products (AGEs) formation and to compare the potency of these extracts with green tea and with the standard antiglycation agent aminoguanidine. When we studied the effects of IP extract on AGE fluorescence generated on bovine serum albumin (BSA) by glycation with methylglyoxal, a dose-dependent effect that reaches 40% at 20 mul/ml of extract was demonstrated. Green tea did not display any significant effect. IP polyphenols are about 2- to 2.5-fold higher in our preparations compared with green tea. The effect of IP, therefore, may be due not only to the higher concentrations but to the different composition in phenolics of the two botanical preparations as well. To better discriminate between an antioxidant or a carbonyl quenching mechanism of action, we explored tryptophan fluorescence and cross-linking by sodium dodecyl sulfate polyacrylamide gel (SDS-PAGE) electrophoresis. The conformational changes induced by glycation and substitution of positive charges in arginine and/or lysine produce a decrease in tryptophan fluorescence. We show that incubation of BSA with methylglyoxal produces dramatic changes in tryptophan fluorescence that are prevented by aminoguanidine. This also prevents the downstream effect of AGE formation. Neither green tea nor IP extracts displayed any significant effect which rules out any significant participation as inhibitors in the first phase of the glycation cascade. The results from the SDS-PAGE serve to confirm the above-mentioned data. The effect is therefore due mainly to an inhibition of the second phase of the glycation reactions, namely the free-radical mediated conversion of the Amadori products to AGE. Taken together our results demonstrate a significant, dose-dependent effect of water extracts of I. paraguensis on AGE adducts formation on a protein model in vitro, whereas green tea displays no significant effect. The inhibition of AGE formation was comparable to that obtained by using millimolar concentrations of the standard antiglycation agent aminoguanidine.