This report describes an efficient chemoenzymatic synthesis of a variety of regioselectively modified beta(1-->4)-oligo- and -polysaccharides. This successfull approach was based on: (i) the use of a "glycosynthase" which is a Glu-197-Ala nucleophile mutant of the retaining cellulase endoglucanase I (Cel7B) from Humicola insolens and (ii) the rational design of modified acceptor and donor molecules through a careful examination of information given by the X-ray structures of wild type and mutated enzymes. The mutant was able to catalyze, in high yield, the regio- and stereoselective glycosylation of alpha-glycobiosyl fluorides both unsubstituted and modified with various mono- and disaccharide accepters, as well as the polymerization of these donors through a single-step inverting mechanism.