Crystals have been obtained of succinylated concanavalin A complexed to a novel bidentate synthetic ligand. The crystals are the first example of a lectin with a synthetic multivalent ligand and the first report of crystallization of succinylated concanavalin A. The crystals were obtained by sitting-drop vapour diffusion equilibrating with a solution of 20% polyethylene glycol, pH 5, 293. 5 K. Crystals are orthorhombic, belonging to space group C2221 with unit-cell dimensions of a = 99.1, b = 127.4, c = 118.9 A. The asymmetric unit contains a dimer, with over 65% of the volume occupied by water. The ligand cross links concanavalin A monomers. Succinylated concanavalin A is known to be a dimer in solution, yet it is found as the typical concanavalin A tetramer in the crystal. The contacts holding together the tetramer appear extensive and suggest that a fine balance between dimer and tetramers exists. Data to 2.65 A have been collected and the structure determined by the molecular replacement method.