Previous studies with the two antibody immunoassay method have demonstrated the presence of an “inhibitor” in fresh plasma which interferes with the precipitating step of the two antibody system. Immunoelectrophoretic studies described here demonstrated no cross-reactivity of rat or human gamma globulin with anti-guinea pig plasma (rabbit source), which had been suggested by other workers. Concomitant inactivation of complement and “inhibitor” by heat, antigen-antibody complex, EDTA, or ammonia indicates that complement is the inhibitor of the two antibody immunoassay system. EDTA (0.01 M) inactivates the plasma “inhibitor,” permitting assay of undiluted plasma by the two antibody method.