400 human sera were tested both in hemagglutination inhibition (HI) and neuraminidase inhibition (NI) tests for antibodies to avian and animal influenza virus subtypes. In the H1 test we only found antibodies to the avian subtype Hav 7 and the animal subtypes Hsw 1 and Heq 2 whereby the latter was mainly demonstrated in elderly persons 60 to 100 years old. The findings of Hav 7 are due to H 3 antibodies and reflect the relationship between both antigens. In the NI test we obtained positive results in 21.8% of the human sera with the neuraminidase subtype N 3 (Nav 2/3) with a peak in persons who were 60 to 70 years old. 11.0% of the sera contained antibodies to the neuraminidase subtype N 8 (Neq 2) and were found exclusively in people 60 to 100 years old, and 9.3% of sera showed positive reactions with the subtype N5 (Nav 5). Until now an immunological relationship between the neuraminidase subtypes N 1, N 2, and N 3 is not known, and could'nt be found in our own studies. Contaminations of antigens can also be excluded. The possible origin of these antibodies to avian neuraminidase subtypes is discussed.