Newcastle Disease of chickens is of interest in comparative pathology because the causal virus has a wide range of pathogenicity and is of only one antigenic type. The virus can be grown to high titre and is easily titrated. Both live attenuated and inactivated vaccines give effective protection under experimental conditions. Vaccination of commercial chickens is associated with variable results, and it is now possible to investigate the factors which complicate vaccination. These include the presence of interfering respiratory infections, immunosuppressive disease of viral origin, variations in the techniques of vaccination and in the programmes of revaccination. Challenge with lethal virus given by aerosol is severe and allows an accurate assessment of the relationship between serum HI levels and the degree of protection. In the later stages of the immune process most but not all protection is associated with the IgG fraction of serum. There is also an increasing amount of evidence to show that local immunity in the respiratory tract is important in the early stages of the immune process.