Abstract: The present invention provides a convenient, efficient method for assaying glycated protein, fructosyl peptide, or fructosyl amino acid which can be performed with reduced effect of fructosyl lysine compounds. The invention also provides a reagent for the assay. The invention is directed to a method for reducing the effect of a fructosyl lysine compound in an assay of fructosyl peptide or fructosyl amino acid contained in a sample, characterized by including causing an enzyme for assaying fructosyl peptide or fructosyl amino acid to act specifically on fructosyl peptide or fructosyl amino acid at a pH of 4.0 to 7.0 and measuring the product at a pH of 4.0 to 7.0; and a method for assaying glycated protein through the above method.