Enzyme urease was immobilized in copolymer matrices of 2-hydroxyethyl methacrylate (HEMA) and N-vinyl pyrrolidone (VP). The activities of immobilized urease stored in phosphate buffer solution of pH 7.0 at 4-degrees-C were examined periodically for up to 90 days. For the matrices of higher VP/HEMA mole ratio in the structure, a higher volume increase and enhanced apparent activity were observed, while HEMA polymer alone proved to have the most stable matrix for prolonged activity. No appreciable amount of enzyme leakage was experienced for any of the matrices prepared. The effective diffusion coefficients of urea through these polymer matrices were calculated with a 'diffusion and reaction' model and the highest effective diffusion coefficient was found with pure HEMA matrix, possibly due to its laminated structure.