Three groups of susceptible chickens were treated with Newcastle Disease vaccine, as follows: group I--orally, strain La Sota; group II--aerosole treatment, strain La Sota and group III--intramusculary, Komarov's vaccine. Varying HI antibody levels were observed following vaccine application. The average antibody titer of group I and II experimental chickens (40 in number for each group) was 25 days post vaccination respectively log2 = 3.47 and log2 = 6.2 while of group III (50 chickens) it was 14 days post vaccination log2 = 8.4. Aerosole challenge with velogenic viscerothropic strain of Newcastle Disease virus caused a sharp change in serum antibody titer. The low antibody titer in group I rose quickly post challenge while in groups II and III, on the contrary, it fell sharply. This characteristic dynamics is proposed for use as an indicator showing the presence of pathogenic virus in vaccinated birds. It was proven that pathogenic Newcastle Disease virus persisted in birds from group I for 40 days and 10% of them were in a state of latent infection (clinically healthy). The virus was isolated after method of organ cultures of tracheal explantates. It was established that the HI antibody level in the blood serum of virus carriers was higher as compared with that of the remaining birds in the group. The use of this fact as an indicator for beginning virusological investigations aiming to reveal latent Newcastle Disease infection is proposed.