An object of the present invention is to provide a peptide derivative for determining β-glucan or endotoxin which allows high sensitivity measurement, and a method for determining β-glucan and/or endotoxin using the same. The present invention relates to (1) a peptide derivative represented by the following general formula [1]:X-A1-Gly-Arg-A2-E-D [1],(2) a reagent for determining β-glucan and/or endotoxin comprising the above-described peptide derivative, (3) a method for determining β-glucan and/or endotoxin, characterized in that a sample containing β-glucan and/or endotoxin, an amebocyte lysate of a horseshoe crab and the above-described peptide derivative are reacted each other, then the resulting released compound represented by the following general formula [2]:H-A2-E-D [2]is separated from unreacted substance and quantified, and the determination is made based on this value, and (4) a reagent kit for determining β-glucan and/or endotoxin, comprising an amebocyte lysate of a horseshoe crab and the above-described peptide derivative as constituents thereof.
