Kerase, a serine protease from Streptomyces fradiae, was immobilized on porous glass (SIKUG(R)) by covalent attachment, through amino groups on the enzyme. Modifications of four lysine residues (44.4% of the accessible or superficial amino groups) results in a loss of 6.5% of the enzymic activity. After immobilization, the optimal reaction pH changed from a range of 7.5-8.5 to 9-10. The immobilized protease was stable in a broad pH range, 6-15 while the soluble protease was irreversibly denaturated at alkaline pHs (pH > 8). The optimal reaction temperature was displaced from 55 to 65 degrees C, showing a higher thermal stability of the immobilized enzyme. Kerase immobilized onto porous glass was stable for at least 28 days, working in a repeated-batch process of three cycles per day, with an activity loss of 22.1 +/- 3.1%.