The synthesis of a polysaccharide from the substrate sucrose was carried out with help of a novel enzyme-membrane reactor. Commercially available capillary pore membranes with diameters of 0.4, 1.0 and 3.0 mu m had to be modified for covalent enzyme immobilization within the pores. This was achieved by heterogeneous photoinitiated graft copolymerization. A grafted poly-(2-amino ethyl methacrylate) layer with glutardialdehyde activation was particularly suited for coupling of fructosyltransferase (FTF, inulinsucrase from Streptococcus mutans), yielding enzyme-membranes with high permeabilities and essentially unchanged FTF activities. However, for a continuous enzymatic reaction driven by trans-membrane substrate flow, it was necessary to use membranes with rather spacious cylindrical pores (3.0 mu m) to avoid their blocking by the produced polyfructan (inulin). The synthesized inulins had a molecular weight between 30 and 50 Mio g/mol with a very low polydispersity.