In order to assess the mechanisms of host resistance to Newcastle disease virus (NDV), the susceptibility of young adult normal, T cell deficient and agammaglobulinemic chickens to an avirulent live vaccine (Bl) and a mesogenic strain of NDV was studied. All animals, regardless of immunological status resisted the vaccine strain. Most normal birds resisted mesogenic NDV, HOWEVER T cell deficient birds were much more susceptible and agammaglobulinemic chickens were extremely susceptible. There was no difference in the kinetics and levels of hemmagglutination-inhibition activity of plasma between normal, control-irradiated and T cell deficient birds nor between dying and surviving birds. Agammaglobulinemic chickens could be partially protected against an otherwise lethal challenge following immunization with avirulent NDV, low doses of mesogenic NDV inoculated intranasally or im injection of beta-propriolactone inactivated NDV mixed in complete Freund's adjuvant. The possible mechanisms for this protection together with the relative roles of humoral, cell mediated and non-specific immunity are discussed.