The defined antibody specificity is to human high molecular weight kininogen. In immunoelectrophoresis, bi-dimensional electrophoresis and radial immunodiffusion (Ouchterlony, Mancini), using various antiserum concentrations against normal plasma, one single precipitin line is obtained. No precipitation is observed against plasma of patients with high molecular weight kininogen deficiency, but its presence can be demonstrated in normal serum. In double radial immunodiffusion the precipitin line obtained against normal plasma shows a reaction of full identity with the precipitin line with purified high molecular weight kininogen. As precipitating antiserum in electroimmunodiffusion, immunoelectrophoresis and radial immunodiffusion. This does not exclude the use of the antiserum in more sensitive techniques if proper controls are included.