Most preferably the most frequent pore-diameter of this support material is between about 250 and about 600 A, in particular between about 300 and about 400 A, especially about 340 A. The most favorable immobilized glucose-isomerase composition is obtained if this support material is contacted with a solution containing from about 20 to about 50, preferably about 25 mg of glucose-isomerase per 1 g