Oligosaccharide subunits were prepared from xyloglucan (XG) by partial hydrolysis with cellulase and added back at micro- to millimolar concentrations to XG in the presence of nasturtium seed xyloglucanase (XG-ase). The oligosaccharides (0.2 mM) stimulated the capacity of this XG-ase to reduce the viscosity of XG solutions by 10- to 20-fold. Purification and fractionation of seed XG-ase activity by gel permeation fast protein liquid chromatography produced a single peak that was much more active in the presence than absence of added XG oligosaccharide. [14C]Fucose-labeled XG nonasaccharide was synthesized by pea fucosyltransferase and shown to be incorporated into polymeric XG in the presence of seed XG-ase without the net production of new reducing chain ends, even while the loss of XG viscosity and XG depolymerization were enhanced. It is concluded that in vitro seed XG-ase can transfer cleavage products of XG to XG oligosaccharides via endotransglycosylation reactions, thereby reducing XG M(r) without hydrolysis. Since this is the only XG-cleaving enzyme that develops in nasturtium seeds during germination, it may be that its transglycosylase and hydrolase capacities are both necessary to account for the rapid and complete depolymerization of XG that takes place.