Fowl were primarily vaccinated with commercial inactivated oil emulsion (10E) Newcastle disease (ND) vaccines and immunity was established by both determination of haemagglutination inhibiting (HI) antibodies and by challenge, over periods similar to the economic life span of the birds. The effect of level of maternal antibodies, route of vaccine administration, and vaccine dose on the vaccination results were studied. In one experiment, immunity provoked by vaccination with 10E vaccine was compared with that obtained by vaccination with live vaccines.