The protective effect of humoral immunity against lethal infection of chickens with Newcastle disease virus was studied. Chickens hatched from eggs laid by hens vaccinated with live attenuated Newcastle disease virus vaccine possessed antibody to various components of the virus, and were resistant to a challenge with a virulent strain of Newcastle disease virus which was 100 per cent fatal for the offspring of nonvaccinated hens. Passive administration of antiserum raised against whole virions provided susceptible chickens protection comparable to that seen in the birds with maternal antibody. When administered passively, both anti-HN serum with virus neutralizing activity, and anti-F serum with only marginal virus neutralizing activity significantly prolonged the survival of infected birds but failed to achieve the level of protection as afforded by the anti-whole NDV serum. The protection provided by the simultaneous presence of anti-HN and anti-F serum was significantly greater than that afforded by either alone and comparable to that of anti-whole NDV serum, indicating the complementary effect of anti-HN and anti-F antibodies not only in cell cultures as reported previously (19), but also in a natural host.