Experimental infection with infectious bursal disease virus (IBDV) at hatching or at 3 weeks of age in White Leghorn chickens without maternally derived antibodies to IBDV resulted in a depression in the antibody response of chickens to Newcastle disease vaccination (NDV) at 4 weeks of age and increased the susceptibility of those birds to challenge with virulent NDV. Infection of non-IBDV immune chickens with IBDV at hatching, but not at 3 weeks of age, also depressed the antibody response of chickens vaccinated at 18, 30, or 42 weeks of age, but had no effect on the susceptibility of those birds to challenge with virulent NDV. Prior exposure to IBDV did not alter disease resistance afforded a bird by NDV vaccination at 18, 30, or 42 weeks of age. However, IBDV infection at hatching did render chickens that were not vaccinated against ND more susceptible to challenge with virulent NDV at 21, 33, or 45 weeks of age than unvaccinated birds which were not infected with IBDV or unvaccinated chickens infected with IBDV at 3 weeks of age.