Administration of live influenza vaccine produces an asymptomatic immunizing infection of the respiratory tract with the appearance of antibodies in the secretions of the respiratory passages.Because of natural variation in the antigenic and biological properties of the influenza virus, new vaccines must be developed every 4-6 years. The appearance of new variants in other countries is a signal to begin converting them immediately into vaccine strains.Strains of virus are usually attenuated for vaccine production by repeated passage through developing chick embryos at 32 degrees C-34 degrees C. In the USSR the attenuated strains are tested for avirulence in human volunteers: no more than 2% of febrile reactions is permissible. Immunogenicity in man is also verified. For use in children further attenuation in chick embryos at 26 degrees C-28 degrees C is required.Some results of field trials of live influenza vaccines in the USSR are discussed.