The wide distribution of broad specificity ß-D-glucan glucohydrolases in higher plants suggests that they play a fundamental role in growth and development. ß-D-Glucan glucohydrolases have been implicated in wall loosening during cell elongation, in wall remodelling, in defense reactions, and in recovery of glucose from different classes of oligosaccharides and polysaccharides. A ß-D-glucan glucohydrolase from barley has been crystallized and its three-dimensional structure solved by x-ray crystallography. On pages 1033–1052, Hrmova et al. describe the interactions between active site amino acid residues of ß-D-glu-can glucohydrolase and two nonhydrolyzable S-substrate analogs to rationalize the broad substrate specificity of the enzyme. Electron density maps of (1,4)-ß-D-glucoside (atoms and density colored in atom colors and green) and (1,3)-ß-D-glucoside (magenta) show that both moieties can be accommodated in the active site of the enzyme. For both S-substrate analogs, the glucopyranosyl residues bound at subsite -1 are in almost identical positions, whereas the relative flexibility of binding at subsite +1, coupled with the projection of the remainder of substrate away from the enzyme's surface, means that the active site can accommodate a range of oligosaccharides and polysaccharides.