Soybean membranes possess high-affinity binding sites for fungal β-glucans that elicit phytoalexin synthesis. The ability of 1,3-1,6-β-glucans, released by acid hydrolysis from mycelial walls of Phytophthora megasperma f.sp. glycinea, to compete for the putative phytoalexin elicitor receptors increases with their average degree of polymerization (DP). The results suggest a function where the probability for glucan fragments of containing a structural determinant that is optimal for binding approaches 1 as the DP tends to infinity. Ligand displacement data obtained against a 125I-labeled glucan elicitor (average DP= 18) provided a theoretical minimum IC50 (50% inhibitory concentration) for 1,3-1,6-β-glucans of 3 nM. The IC50 value obtained for a synthetic hepta-β-glucoside having a known elicitor-active structure was 8 nM, remarkably close to the predicted value. Displacement of the 125I-glucan of large DP was uniform and complete showing that the heptaglucoside had access, with similar affinity, to all sites available to the radioligand. Further analysis using a 125I-labeled aminophenethylamine derivative of the heptaglucoside suggested that the putative glucan-elicitor receptors bind a basic structural determinant present in all elicitor-active glucans from the soybean pathogen P. megasperma. © 1990.