Poretz and Goldstein showed that X-phenyl beta-D-glucopyranosides prevent the agglutination of concanavalin A with polysaccharides and derived inhibition constants for the process. Using their data the binding of 25 glucosides to concanavalin is now shown to be correlated with the molar refractivity of the substituents on the phenyl ring. This is interpreted to mean that it is the bulk of the substituents and not their hydrophobicity which prevents the union of concanavalin and the polysaccharide. These results are similar to those found for other haptens preventing antibody-antigen interaction.