Abstract Structure-activity relationships among glycoside activators of ryegrass (Lolium multiflorum) (1–<3)-β-glucan synthase were investigated using a number of natural and synthetic glycosides, including some carrying photoaffinity functions. There is an absolute requirement for a β-D-glucosyl moiety in the activator, both S- and N-glucosides are active, and the position of the glucosidic linkage in β-glucose disaccharides has a significant effect on the affinity of binding. However, the binding requirement does not extend beyond a single β-D-glucosyl residue, and β-D-oligoglucosides are less effective than disaccharides. The nature of the aglycon has a major influence on the binding affinity. Hydrophobic aglycons lower the concentration required for half-maximal stimulation of the enzyme obtained from an Eadie-Hofstee plot of kinetic data (Ka) for activation, but charged aglycons increase Ka. Relative to methyl-β-D-glucoside and cellobiose (Ka 1.1 mM), the most potent compounds tested were N-[4-(benzoyl)benzoyl]-β-D-glucosylamine and 2[prime]-[4-azidosalicylamino]ethyl-1-thio-β-D-glucoside with Kas of approximately 30 [mu]M. The latter also was tested for its potential to specifically label the β-glucoside-binding site on the synthase, but under the conditions used the binding was found to be nonspecific.