Comparative studies of the antigenic structure of influenza B viruses isolated in 1976-1977 and in previous years demonstrated one-way relationship of the hemagglutinin of B/76 viruses with that of B/72 viruses. Antisera to new B/76 strains neutralized the hemagglutinating activity of all known reference influenza B virus strains beginning from 1972. Influenza B/76 viruses were divided into two groups depending on complete or partial neutralization by antiserum B/Hong Kong/7/75. The study of the antigenic structure of neuraminidase showed no significant differences between influenza B/76 and B/72 virus neuraminidase. All the influenza B viruses from different years under study had marked interferon-inducing properties and were sensitive to interferon. Unlike the Yamagata/1/73 strain, all the influenza B/76 viruses under study were resistant to thermostable inhibitors of normal horse, guinea pig, and rabbit sera.