The capacity of salt-soluble enzymic extracts of young green tomato fruit to reduce the viscosity and molecular size of tamarind xyloglucan (XG) in solution is stimulated markedly by the addition of XG subunit oligosaccharides. This is not due to end-product activation of an endo-1,4-β-glucanase or XGase, for the following reasons: non-specific endo-1,4-β-glucanase (carboxymethylcellulase) activities in developing tomato fruit were recovered from buffer and detergent-extracts but were barely detectable in subsequent extracts with 1 M NaCl, where XG-depolymerizing activity predominated. Salt-soluble extracts alone were totally unable to degrade high Mr XG from tamarind seed in the absence of fragments of low Mr XG or oligosaccharide subunits. The degradation catalysed by tomato salt extract plus tamarind XG and XG oligosaccharides failed to proceed to completion or generate low Mr end products, as happens in the presence of cellulase. The reactions progressed with no detectable net increase in total reducing power of the reaction mixtures, but were accompanied by condensation of XG cleavage fragments with XG [ 14C]oligosaccharide, as expected from XG endotransglycosylase (XET) activity. The activity leading to the depolymerization of XG, as assayed viscometrically in the presence of XG oligosaccharide, co-chromatographed in various media in the same fractions as XET activity, as assayed radiometrically. It is concluded that XET activity in salt extracts of young green tomato is sufficient by itself to explain the observed capacity of extracts to depolymerize tamarind XG. In vivo, 1,4-β-glucanase or XGase activity would also be required to generate the low Mr XG acceptors needed for transglycosylase to catalyse a net depolymerization of XG.