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ϵ-Poly-L-lysine (ϵ-PL) exists as a cationic polymer in acidic solution. Yellow tetravalent 12-molybdosilicate ([SiMo12O40]4−) anion is formed in acidic solution containing and anions. By mixing the test ϵ-PL solution with the acidic − solution, the [SiMo12O40]4− anion associates with the cationic ϵ-PL to form yellow precipitate. The yellowness of the supernatant decreased with the initial concentration of ϵ-PL in the test solution due to the residual [SiMo12O40]4− anion. By addition of ascorbic acid to the supernatant, the residual [SiMo12O40]4− anion was reduced to blue molybdosilicate species. Thus the concentration of ϵ-PL in sample media can also be estimated by the blueness of the reaction mixture. The present method can be applied successfully to the screening of ϵ-PL-synthetase in culture media. [ABSTRACT FROM AUTHOR]