Chicks vaccinated with live Hitchner B1 Newcastle disease vaccine at 17 days old and subsequently re-vaccinated with an oil emulsion killed Newcastle disease vaccine at either 38 or 52 days old showed high and persistent HAI antibody levels for at least eight months. Re-vaccination of these birds at 17 weeks old caused a further rise in antibody level to log212 which, even at 38 weeks, had dropped only to log210. Chicken primarily vaccinated with oil emulsion killed vaccine at six weeks old developed HAI antibody levels after four to five weeks of log29 which re-vaccination four weeks later increased to log211. Chicken given killed aluminium hydroxide adjuvant Newcastle disease vaccine were serologically HAI negative 13 weeks after vaccination while those given the oil emulsion vaccine still showed an antibody level of log28. Groups of birds inoculated with oil emulsion vaccine and then, at 20 weeks old, challenged with virulent Newcastle disease showed a 100 per cent survival rate. The particular merits of oil emulsion killed Newcastle disease vaccine for laying and breeding birds are discussed.