Examinations of 842 blood serum specimens from children varying in age from 0 to 14 years and 975 blood serum specimens from adults of 20-60 were carried out. The infants of 0-6 months of age may be considered to be immunologically protected by passively transmitted maternal antibodies. The formation of active immune response occurs at the age of 6 months to 14 years with subsequent antibody persistence and additional latent immunization in older age. Children of 6 months to 7 years are most susceptible to infection with influenza C virus. Among the adult USSR population, antibody to influenza virus was found in approximately 85.7% of subjects. The proportion of seropositive subjects varied in different parts of the country. The greatest numbers of positive findings and high levels of antibody to influenza C virus were found in the adult and child population of Moscow.