Fibrinogen is synthetised in the parenchymatous cells of the liver. It is broken down in the plasma by cells equipped with the appropriate enzymes. The fibrinogen molecule is a dimer with a molecular weight of 340,000. Each monomer consists of three polypeptidie chains (alpha A, beta B and gamma) joined by disulphide bridges. The N-terminal region is rigid and constitutes the N-DSK (N-disulphide knot) around which the three C-terminal free extremities are rolled, thereby giving a symmetry and spherical form to the molecule. The existence of several types of alpha A chains explains their important role in polymerisation.