The synthesis of a polysaccharide from the substrate sucrose was carried out with help of a novel enzyme-membrane reactor. Com. available capillary pore membranes with diams. of 0.4, 1.0 and 3.0 mm had to be modified for covalent enzyme immobilization within the pores. This was achieved by heterogeneous photoinitiated graft copolymn. A grafted poly-(2-amino Et 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 enzymic reaction driven by trans-membrane substrate flow, it was necessary to use membranes with rather spacious cylindrical pores (3.0 mm) to avoid their blocking by the produced polyfructan (inulin). The synthesized inulins had a mol. wt. between 30 and 50 Mio g/mol with a very low polydispersity.