The immune response of the chicken to vaccination against Newcastle disease with live virus and killed emulsified virus in relation to the length of time between two vaccinations.

Chicks with no specific antibodies were initially y vaccinated against Newcastle disease at 21 days old with the live 'La Sota' strain or beta-propiolactone-killed virus emulsified in mineral oils. After a second vaccination 15 or 21 days later with the live "La Sota' strain, the interference produced by the first vaccination on the antibody production induced by the second vaccination was investigated. It was found that the live virus vaccine used for the second immunization was affected by the antibodies induced by the first vaccination, as shown in previous work utilizing 1-day-old chicks.