Pectinex Ultra SP-L immobilized magnetic duolite-polystyrene particles were used in a magnetic fluidized bed reactor. Enzyme activity was determined by the refractive method. The maximum activity of the enzyme in immobilized form was observed at 55 degrees C and pH 5. It was found that an increase of the flow rate of the substrate caused a decrease in the enzyme activity. Michaelis-Menten constant and maximal reaction rate were found as 0.439 g L-1 and 44.8 g L-1 s(-1), respectively for the free enzyme. The apparent values of these terms for the immobilized enzyme were calculated as 4.416 g L-1 and 5.5 x 10-5 g s(-1) g particles(-1), respectively. Although the activity of the immobilized enzyme was increased by the increase of the reaction temperature up to 55 degrees C, the reaction stability was found poor at 55 degrees C. (c) 2004 Published by Elsevier Ltd.