All six above-mentioned conservative amino acids were found only in N9 NA. Avian influenza virus NAs of other subtypes usually lack lysine 432, but its absence does not interfere with their haemoadsorption activity. Human influenza virus neuraminidases usually lack the HB-site a.a. sequence. At the same time, two early isolates of human influenza viruses of the Н2N2 subtype (RI+/57 and A/Leningrad/ 134/57) have the HB-site "frame" (serine triplet and tryptophan) [10, 11], which might be indicative of elimination of this site in the course of the influenza virus adaptation to replication in humans.