Studied were immunoelectrophoretically the antigenic properties of 16 virulent and 4 vaccinal strains of the Newcastle disease virus--B1, La Sota, Komarov, and Roakin. Results showed that all virulent strains isolated in the course of 5 years had the same antigenic components as compared both to each other and to the vaccinal strains. This similarity was manifested by the presence of a clearly expressed precipitation line found with all strains in one and the same place of the immunoelectrophoregrams. It was assumed on the basis of the results obtained that there is no variation in the antigenic structure of the Newcastle disease virus. This confirmed the appropriateness in using the vaccinal strains B1, La Sota, Komarov, and Roakin in the active prophylaxis of birds against Newcastle disease.