The seeds of Vicia cracca are known to contain two lectins: an N-acetylgalactosamine-binding lectin, which reacts specifically with human erythrocytes of blood group A, and a mannose/glucose-binding lectin unspecific towards human blood groups. The second lectin is the object of the present work. Purification was achieved by affinity chromatography on Sephadex G-100. The native lectin has a molecular weight of 44,000. It is composed of two small (Mr 5,700) and two large subunits (Mr 17,500). Binding of sugars to the lectin was assayed by equilibrium dialysis and spectrophotometrically; the association constants for glucose are 1.38 X 10(3) l/mol and 2.5 X 10(2) l/mol respectively. The primary structure of the small subunit was determined by analyses of the whole chain, of tryptic and CNBr peptides. It was shown to consist of 53 amino acid residues. The mannose/glucose-binding lectin from Vicia cracca is highly homologous to the lectins from Vicia sativa, Vicia faba, Pisum sativum and Lens culinaris. There is, however, only limited homology of the N-acetylgalactosamine-specific lectin from Vicia cracca.