Norakin-resistant (NR) mutants of fowl plague virus (A/FPV/Weybridge, H7N7) have 1 to 2 (in one instance 3) amino acid substitutions in different positions of the heavy (HA 1) and/or light (HA 2) subunits of the haemagglutinin (HA) molecule. Investigation of NR mutants using the haemagglutination inhibition test with monoclonal antibodies (MAb) to the HA of A/seal/Massachusetts/80 (H7N7) virus revealed that one of the mutants (NR 1) differs antigenically from the wild-type fowl plague virus: its haemagglutination was not inhibited by MAb 55/2 and 58/6. By contrast, MAb-resistant (escape) mutants, selected from the wild-type fowl plague virus under pressure from MAb 55/2 or 58/6, showed reduced drug sensitivity. These findings suggest a possibility of correlation between alteration of influenza virus antigenicity and change of its sensitivity to drugs whose target is the haemagglutinin. This potential effect should be taken into account when antiviral substances directed to surface influenza virus antigens are being developed for use as antiviral drugs.