The effect of cross-link density and amount of porogen on physical properties of the glycidyl methacrylate-pentaerythritol triacrylate (GMA-PETA) copolymer and intern on the immobilization of milk clotting protease from Aspergillus niger MC4 was studied. Almost quantitative expression of the milk clotting protease was achieved with copolymer of 251% cross-link density and generated with 65.4 mt of cyclohexanol as porogen. The pH of 4.0 and 18 h period were the best conditions for binding the milk clotting protease onto the polymer. The immobilized milk clotting protease prepared had an activity of 6000 units/g. The optimum pH and temperature ranges for both soluble and immobilized milk clotting protease preparations were pH 5.6-5.8 and 55-60 degrees C, respectively. Immobilized milk clotting protease was more stable than the soluble enzyme; the half-lives at pH 4.0 and 37 degrees C were 160 and 60 days, respectively. At pH 5.8 and 37 degrees C, the parameters favorable for milk clotting, the immobilized enzyme preparation did not exhibit any aspartic protease activity.