A purified alkaline thermo-tolerant lipase from Pseudomonas aeruginosa MTCC-4713 was immobilized on a series of five noble weakly hydrophilic poly(AAc-co-HPMA-cl MBAm) hydrogels. The hydrogel synthesized by copolymerizing acrylic acid and 2-hydroxy propyl methacrylate in a ratio of 5 : 1 (HG(5:1) matrix) showed maximum binding efficiency for lipase (95.3%, specific activity 1.96 IU mg(-1) of protein). The HG(5:1) immobilized lipase was evaluated for its hydrolytic potential towards p-NPP by Studying the effect of various physical parameters and salt-ions. The immobilized lipase was highly stable and retained similar to 92% of its original hydrolytic activity after fifth cycle of reuse for hydrolysis of p-nitrophenyl palmitate at pH 7.5 and temperature 55 degrees C. However, when the effect of pH and temperature was studied on free and bound lipase, the HG(5:1) immobilized lipase exhibited a shift in optima for pH and temperature from pH 7.5 and 55 degrees C to 8.5 and 65 degrees C in free and immobilized lipase, respectively. At 1 mM concentration, Fe3+, Hg2+, NH4+, and Al3+ ions promoted and Co2+ ions inhibited the hydrolytic activities of free as well as immobilized lipase. However, exposure of either free or immobilized lipase to any of these ions at 5 mM concentration increased the hydrolysis of p-NPP (by similar to 3-4 times) in comparison to the biocatalysts not exposed to any of the salt ions. The study concluded that HG(5:1) matrix efficiently immobilized lipase of P. aeruginosa MTCC-4713, improved the stability of the immobilized biocatalyst towards a higher pH and temperature than the free enzyme and interacted with Fe3+, Hg2+, NH4+, and Al3+ ions to promote rapid hydrolysis of the substrate (p-NPP). (c) 2006 Wiley Periodicals, Inc.