An enzymatic reaction rate of glucose oxidase (GOD) using ferricyanide ion ([Fe(CN)₆]³⁻) as an oxidant significantly increased by the addition of ε-poly-L-lysine (ε-PL). The bimolecular rate constant between GOD and [Fe(CN)₆]³⁻ in the presence of ε-PL reached about 10000-fold relative to what it was measured without the ε-PL, and the Michaelis constant decreased. The reaction rate reached a maximum at around pH 6, where ε-PL and GOD possess highly positive and negative charges, respectively. The increment of the reaction rate by ε-PL can be attributed to the electrostatic association of the polycationic ε-PL with the negatively charged GOD to form a polyion complex soluble in the aqueous medium. The adduction of the cationic polymer may relieve the electrostatic repulsion between GOD and [Fe(CN)₆]³⁻, so that the electron transfer effectively occurs between them.