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We describe a new colorimetric assay method using glucose oxidase (GOx) to detect ε-poly- l -lysine (εPL). This method uses εPL's remarkable effect of promoting the enzymatic reaction of GOx with ferricyanide ion. This reaction reduces ferricyanide ion to ferrocyanide ion, accompanied by a color change from yellow to colorless. In this colorimetric assay, the detection limit of εPL was estimated to be approximately 0.5 mg/L when purified εPL samples were used. εPL has usually been produced by a fermentation process using Streptomyces albulus species. The components of the culture broth showed interference effects against the assay method. However, due to the high sensitivity of the assay method for εPL, εPL could be detected in the culture broth without any pretreatment. The detectable concentration of εPL in the culture broth, c PL,ac , was estimated to be approximately 20 mg/L. By combining the Berlin blue reaction with this method, the c PL,ac was reduced to 10 mg/L. In light of the proposed method's simplicity and sensitivity, it could be useful for screening εPL synthetic enzymes and microorganisms. [ABSTRACT FROM AUTHOR]