Activated fibers were used as a new support material for the immobilization of horseradish peroxidase (HRP). Poly(ethylene terephthalate) (PET) fibers were grafted with glycidyl methacrylate (GMA) using benzoyl peroxide (Bz(2)O(2)) as initiator. 1,6-diaminohexane (HMDA) was then covalently attached to this GMA grafted PET fibers. HMDA-GMA-g-PET fibers were activated with glutaraldehyde and HRP was successfully immobilized. Both on the free HRP and the immobilized HRP activities, pH, temperature, thermal stability, and reusability were investigated. Both free enzyme and immobilized enzyme were used in a batch process for the degradation of azo dye. About 98% of azo dye removal was observed with immobilized HRP, while 79% of azo dye removal was found with the free HRP. 45 min of the contact time is sufficient for the maximum azo dye removal. The HRP immobilized on modified PET fibers were very effective for removal of azo dye from aqueous solutions.