Antineuraminidase and antihemagglutinating antibody studies were carried out in parallel in sera from subjects born in Bulgaria in 1968-1972, 1956-1960, 1946-1950, 1925-1935 and 1917-1920. It was found that the amount of both antineuraminidase and antihemagglutinating antibody in sera from normal subjects could vary depending upon the year of birth and the strain used for the test. The antibody spectrum was most narrow in children under 4 and wider in subjects born before 1925. Serograms of the age distribution of antineuraminidase antibody were different in 4 out of 5 strains used which confirmed the hypothesis of Francis and Davenport of the more solid immunological response to the first exposure to virus. Parallel examinations of the antibody spectrum in human sera confirmed differences in the antigenic specificity of the "active center" of neuraminidases of A/Hong Kong/1/68 (H3N2) and A/Singapore/1/57 (H2N2) viruses and for the first time provided evidence attesting to the existence of similar strain differences in neuraminidases belonging to the N1 group. Examinations for antineuraminidase antibody in addition to antihemagglutinins increase the effectiveness of evaluation of the immunological structure of the population.