Studies were carried out on the immune response of birds-parental forms of broilers--in the combined use of an inactivated vaccine Dessau and a live avirulent vaccine La Sota. The dynamics was followed up till the age of 58 weeks of the changes taking place at an antidody level, and the resistance of the birds after challenging with a virulent virus. After the first immunization with a killed vaccine there was an average geometric titer of 78.65, and the resistance of the birds was low. The revaccination with an inactivated vaccine caused an increase in the amount of antibodies and a rise in the level of resistance. In the immunization with a live La Sota vaccine there was an abrupt rise of the immune titer--214.31 (in the twenty-eighth week), and 207.60 (in the thirty-eighth week). During the following periods there was a gradual drop of the antibody level, however, this was accompanied by a comparatively high resistance of the birds at challenge. The results obtained are in relation to some immunogenic properties of the experimental vaccine Dessau the use of which develops an immunogenic background at which, with a two-fold reimmunization applying a live vaccine La Sota the immunogenesis process is strongly enhanced.