The light chain of human high molecular mass kininogen consists of 255 amino acid residues. The half-cystine residue which forms the single disulfide bridge to the heavy chain is located in position 225. The light chain contains 9 O-glycosidically linked carbohydrate side chains. A comparison of the human high molecular mass kininogen light chain with the bovine high molecular mass kininogen light chain reveals a pronounced homology. However, in position 88 of the human sequence an insertion of 22 amino acid residues was found. This insertion and an amino acid exchange in position 131 may explain the different behaviour of human and bovine high molecular mass kininogen during plasma kallikrein digestion.