Abstract: In order to study the rule of antibody fluctuation and stability of Newcastle disease antibody activity in serum after immunization with inactivated Newcastle disease vaccine, the hemagglutination inhibition test was used to determine the titers of Newcastle disease antibody in serum of chickens immunized with Newcastle disease virus and avian influenza virus （H9 subtype） dual inactivated vaccine. The titers of Newcastle disease antibody in serum treated by different methods such as room temperature, 4 ℃ storage, -20 ℃, -80 ℃ freeze-thaw and heating were determined. The results showed that the average titer of Newcastle disease antibody reached 8.1 log₂ at 14 d and 8.9 log₂ at 35 d after the second immunization. The antibody titer remained 7.8 log₂ at 70 d after immunization, and the antibody titer was 7.8 log₂ to 8.9 log₂ from 14 d to 70 d after the second immunization. The serum Newcastle disease antibody was relatively stable at room temperature and 4 ℃, and the titer of Newcastle disease antibody was almost not affected by room temperature for 13 days. However, the titer of Newcastle disease antibody was affected by 4 ℃ for 9 days, and the difference was significant （P<0.05）. The titer of Newcastle disease antibody was not affected by 56 ℃ for 30 min. Repeated freeze-thaw for three times had a significant effect on the antibody titer of Newcastle disease （P<0.01）. The results showed that the level of Newcastle disease antibody was high and the maintenance time of the high level of Newcastle disease antibody was long after the broilers were immunized with inactivated vaccine twice at 1 and 21 days of age. Newcastle disease antibody of chicken blood could resist heating at 56 ℃, and its activity was stable at room temperature. Repeated freezing and thawing could affect its titer.

Key words: Newcastle disease, hemagglutination inhibition test, antibody titer, inactivated vaccine, antibody stability, preservation of serum