A covalent immobilization of urease was conducted on carboxylic cation-exchange membranes (CEM) prepared by radiation-initiated graft copolymerization of acrylic acid (AA) on polyethene (PE) thin films. Six types of CEM with different grafting degree (from 26.5 to 95.2%) were used as carry. The carboxyl groups were activated by the carbodiimide method in order to carry out a covalently immobilization. The amount of bound protein and the enzyme activity were determined in each immobilized system. It was established that the urease, immobilized on CEM with 64.2% grafting degree, featured the highest relative activity - 80.32%. The amount of bound protein on this membrane type was 6.01 mg/cm(2). The basic characteristics of the immobilized and the free enzymes were determined (pH(opt), T-opt and pH(stab)). It was found out that the immobilized urease had greater thermal and storage stability in comparison with the free enzyme. It was proven that CEM with a grafting degree of 64.2% would be a suitable carrier for urease immobilization.