Influenza viruses causing epidemics in the U.S.S.R. in 1968-1982 and 1983 were analysed in the lectin test (LT) using polyclonal and monoclonal antibodies with specificity against neuraminidase (NA) of N2 subtype. Heterogeneity of the U.S.S.R. virus isolates in the reaction with monoclonal and polyclonal antibodies was demonstrated, though they were coming from the same year epidemic. The LT turned out to be an appropriate method to detect the antigenic drift in influenza virus NA. The results of LT were in a good agreement with those obtained by colorimetric estimation of the inhibition of enzyme activity and by competitive solid phase radioimmunoassay (SP RIA). In the LT only low steric inhibition of the NA with antihaemagglutinin monoclonal antibodies (MoAb) reacting in haemagglutination inhibition and virus neutralization tests was detected.