A crude preparation of horseradish roots was used as a low-purity source of horseradish peroxidase (HRP) in dye decolorization experiments. The technical feasibility of the process was studied in bench scale for enzymatic removal of acid orange 7 (AO7), a synthetic dye. Further studies were carried out to understand the effects of process parameters such as pH value, H(2)O(2) level, concentrations of the synthetic dye, and HRP during enzyme-mediated dye degradation. Experimental data revealed that the concentration of AO7, pH of the aqueous phase, amount of the enzyme, and H(2)O(2) level played significant roles on the overall enzymatic reaction. Polyethylene glycol, as an anti-inactivation of HRP, in various concentrations showed no significant effect on the decolorization. The experimental data of initial reaction rates were fitted using an analytical equation proposed by Michaelis-Menten. The acute toxicity tests using Daphnia magna exhibited that the enzymatic treatment significantly decreased the toxicity of the dye solution.