The invention relates to enzymatic compositions containing intra-cellular glucose-isomerase which is enclosed in structures based on cellulose ester, their preparation and their use for isomerizing glucose into levulose.In addition to the cellulose ester and micro-organism cells, e.g. Streptomyces phaeochromogenes, these compositions contain a sparingly water-soluble magnesium compound and optionally a sparingly water-soluble cobalt compound.These compositions make it possible to isomerize glucose into levulose in a continuous process, with the addition of either no, or a small amount of magnesium ions to the glucose syrup to be isomerized and without the addition of cobalt ions.