The reactivity of the antiserum is restricted to C4 binding protein. In immunoelectrophoresis and radial immunodiffusion, using various antiserum concentrations against human plasma a single precipitin line is obtained and shows a reaction of full identity with the purified immunogen. In precipitating techniques as immunoelectrophoresis and single and double radial immunodiffusion (Mancini, Ouchterlony) to identify the presence of C4 binding protein or to determine its concentration. The presence of non-precipitating antibodies has not been assayed. This does not exclude the use of the antiserum in non-precipitating antibody-binding techniques if proper controls are included.