This paper presents the results of urease immobilization onto methacrylic acid-acrylamide grafted poly(ethyleneterephthalate) fibres. The graft yield strongly affected the maximum activity of the immobilized enzyme up to a value of 70.2%. Higher grafting caused a decline in urease activity and led to the degradation of the fibres. The minor changes observed in K-m and V-max demonstrated that the conformational changes existed during immobilization were not extensive. However, 70.2% methacrylic acid-acrylamide-g-fibres containing urease were more stable towards acidic and alkaline pH, high temperature and storage conditions compared with free enzyme. Apart from the increase in stability to heat inactivation, the initial enzymatic activity of the urease-fibre system remained almost unchanged even after 40 repeated assays corresponding to 10 h of operation in 4 months, indicating the excellent durability of the system.