Hapten inhibition measurements on the precipitin reaction between Pisum sativum lectin and Pichia pinus phosphomannan showed the lectin to bind D-mannose, D-glucose, D-fructose and L-sorbose. Unmodified hydroxyl groups at the C-4 and the C-6 positions of the D-glucopyranose ring were essential for binding to the protein. Modification of the C-2 hydroxyl group was allowed in the D-glucopyranose ring but not in the D-mannopyranose configuration. Substitution of the hydroxyl hydrogen atom at the C-3 position of D-glucose increased the binding efficiency. With the exception of gentiobiose, the beta-linked glycobioses tested were not bound to the lectin, whereas the alpha-linked glycobioses were potent inhibitorsmin general, the P. sativum lectin was found to be less sensitive to structural variation of inhibiting carbohydrates than concanavalin A, the lectin from Canavalia ensiformis.