An inactivated oil-based Newcastle disease vaccine was prepared using Mukteswar vaccine strain. The virus was propagated in 10-day old embryonating eggs and inactivated by 0.12% formalin for 48 hours at 37 degree C. The vaccine was formulated with 1 part antigen (aqueous phase) and 4 parts oil base. The oil base contained Tween-80 1%, Arlacel-A 10% and Mineral oil 89%. The stability of the vaccine was found satisfactory after 6 months and its viscosity and injectability was fairly ideal. The antigenicity of the vaccine was determined in 16 week-old pullets. The seromonitoring of the vaccinated and the control pullets was carried out for three months post- vaccination by Haemagglutination inhibition (HI) test. Blood samples were taken at fortnightly intervals. The Geometric mean HI titre of the vaccinated pullets on the day of vaccination 15, 30, 45, 60, 75, 90, 105 and 120 days post- vaccination was 18.4, 4.9, 87.5, 192.3, 257.6, 111.4, 91.7, 63.9 and 30.0. However, in non-vaccinated control pullets it was found to be 18.4, 3.7, 3.7, 4.3, 3.8, 4.0, 3.5, 2.8 and 2.3 respectively. The inactivated oil-based vaccine induced a marked antibody response which continued upto three months.