The reactivity of the antiserum is restricted to fibrinogen. In immunoelectrophoresis and radial immunodiffusion (Ouchterlony), using various antiserum concentrations against fresh normal human plasma a single precipitin line is obtained which shows a reaction of identity with the precipitin line obtained with purified fibrinogen. No reaction is obtained with any other plasma protein component or serum. However, the antiserum may also react with fibrin monomers, circulating fibrinopeptides and fibrin degradation products. In precipitating techniques as immunoelectrophoresis and single or double radial immunodiffusion to identify the presence of fibrinogen in human plasma or other body fluids or to determine its concentration. The normal concentration of fibrinogen in the blood is 2.5 to 3.5 ml /ml, but lower levels are usually adequate for haemostasis. In newborn infants the value is 1.2 to 2.4 mg/ml. Synthesis of foetal fibrinogen may persists for up to 8 weeks, when adult levels are reached. Adult level increase with age and are a risk for heart disease, myocardial infarction and stroke. Fibrinogen is an acute Phase protein and increased levels are found in loosing enteropathies, in severe malnutrition, in tissue necrosis and in malignancy. Extremely high levels are seen in acute pancreatitis and, to a lower extent, in nephritic syndrome. A moderate raise may also been seen during pregnancy and the use of oral contraceptives. Fibrinogen deficiency may be congenital or acquired. If sufficiently severe, it may result in a bleeding disorder. The congenital form is very rare. Acquired hypofibrinogaemia is relatively common, probably indicating increased consumption during intravascular clotting.