The reactivity of the antiserum is restricted to C5. 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 C5 protein. No reaction is obtained with any other plasma protein component or serum. In precipitating techniques as immunoelectrophoresis and single or double radial immunodiffusion to identify the presence of C5 containing complexes in human plasma or other body fluids 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 more sensitive non-precipitating antibody-binding techniques if proper controls are included.