The seeds of winged bean, Psophocarpus tetragonolobus (L.) DC, contain a group of acidic lectins (pI approximately equal to 5.5) and a group of basic lectins (pI greater than 9.5) characterised by different erythrocyte hemagglutinating specificities. Three basic lectins were separated and purified to apparent homogeneity by chromatography on Ultrogel AcA44 and SP-Sephadex C-25. These lectins are glycoproteins with relative molecular mass of 58 000. The total carbohydrate content of the major lectins, B2 and B3, was 5% and was comprised of mannose, N-acetylglucosamine, fucose and xylose in amounts corresponding to 9, 8, 8 and 3 mol/58 000 g, respectively, for both lectins. Electrophoresis in dodecyl sulfate gave a single subunit of relative molecular mass 29 000 for each lectin. Isoelectric focusing in 8 M urea revealed two differently charged subunits in each of the isolated lectins; a total of three different subunits in the three lectins was observed. The basic lectins have essentially the same amino acid composition and contain no half-cystine. Two of the lectins (B2 and B3) showed identical amino-terminal sequences and the sequence of lectin B3 to residue 40 revealed extensive homology with other legume lectins such as soybean lectin. The basic lectins agglutinated trypsinized rabbit and trypsinized human (type A and B) erythrocytes but not trypsinized human (type O) erythrocytes. The lectins were inhibited by N-acetylgalactosamine, D-galactose and D-galactose containing disaccharides and trisaccharides. The specificity appears to be directed to glycosides having non-reducing (terminal) alpha-D-galactopyranosyl groups. beta-D-Galactosides, such as lactose, are poor inhibitors.