The antiviral activity of Newcastle disease immune sera.

The diagnostic value of the hemagglutination inhibition test for Newcastle disease has been established for some years (Brandly et al., 1946). The question of the relationship of the hemagglutination inhibition activity (HI) of immune sera to virus-neutralizing activity (SN) or to the refractivity of the host to challenge infection is still one of both fundamental and practical importance. Data have been gathered in this laboratory that make it difficult to ascribe the various expressions of antiviral activity to a single antibody.