Pullets about to enter a laying flock were revaccinated by intramuscular injection of live Mukteswar strain Newcastle disease virus, a procedure commonly used in Pakistan. Chicks hatched from eggs produced 24 weeks later were assessed for passively acquired immunity. Levels of haemagglutination-inhibition (HI) antibody and resistance to challenge with velogenic Newcastle disease virus were determined on groups of 50 chicks at 1, 4, 7, 10, 13, 16, 19, 22 and 25 days of age. Levels of maternally derived antibody declined from a geometric mean titre of 40 (2 5.3) on the day of hatching to less than two (2 0.3) at 25 days of age, with a decline of 1log 2 in about 5 days. Mortalities after challenge varied from 18% at 1 day of age to 90% at 25 days of age. Survival was significantly related to the level of antibody at the time of challenge. None of 111 chickens with an antibody titre of 40 (2 5.3) or more succumbed to challenge. Mortality rates varied from 78.6% in chicks with no detectable antibody at challenge to 27.6% in chicks with a titre of 20 (2 4.3). Levels of HI antibody and neutralizing antibody were strongly correlated.