As part of a strategy to determine the precise role of pea (Pisum sativum) lectin, Psl, in nodulation of pea by Rhizobium leguminosarum, mutations were introduced into the genetic determinant for pea lectin by site-directed mutagenesis using PCR. Introduction of a specific mutation, N125D, into a central area of the sugar-binding site resulted in complete loss of binding of Psl to dextran as well as of mannose/glucose-sensitive haemagglutination activity. As a control, substitution of an adjacent residue, A126V, did not have any detectable influence on sugar-binding activity. Both mutants appeared to represent normal Psl dimers with a molecular mass of about 55 kDa, in which binding of Ca2+ and Mn2+ ions was not affected. These results demonstrate that the NHD2 group of Asn125 is essential in sugar binding by Psl. To our knowledge, Psl N125D is the first mutant legume lectin which is unable to bind sugar residues. This mutant could be useful in the identification of the potential role of the lectin in the recognition of homologous symbionts.