Abstract A 13·6 kDa protein from corn seeds is known to be a highly selective inhibitor of human blood coagulation Factor XIIa (or activated Hageman factor). We have crystallized this inhibitor at 23°C and pH 7·5 from a solution of 30% polyethylene glycol 400, 0·2 M MgCl 2, and 0·1 M Hepes. The crystals diffract to at least 2·1 Å resolution. The space group is P4 22 12 with a = b = 57·15 Å and c = 80·5 Å. The crystals contain 51% solvent. Two heavy atom derivatives have been identified.