Trypsin can be obtained from waste disposal of Nile tilapia (Oreochivinis niloticus) intestine, the most important fish species in Brazilian aquaculture. This protease was covalently immobilized on ferrornagnetic Dacron (polyethyleneterephthalate or PET). Dacron film was converted to Dacron-hydrazide powder and further magnetized. Then the enzyme was covalently bound to the magnetic particles. The protein amount and specific activity of the immobilized enzyme on 0.6 mM BAPNA (pH 8.0 at 25 degrees C were 25.6 mg/g of particles and 18.5 +/- 0.253 mU/mg protein (29 +/- 1%) of that estimated for the soluble enzyme), respectively. The derivative showed an apparent K (0.132 +/- 0.044 mM) and optimum pH (7.0) lower than those found for the soluble enzyme (0.735 +/- 0.141 mM and 8.0). The enzyme was inhibited by benzamidine and TLCK (typical trypsin inhibitors) and metallic ions, especially aluminium and copper. This water insoluble enzyme was stable during about two months stored at 10 degrees C and can be reused. (c) 2005 Elsevier Ltd. All rights reserved.