Antigenic variation of the neuraminidase of A/H3N2 influenza viruses may be associated with modifications of the catalytic activity of this enzyme. We observed this phenomenon when studying two prototype strains: A/Hong Kong/1/68 (X31K) and A/Bangkok/2/79. For the neuraminidases of these strains, we determined their substrate specificity, initial velocity, optimum pH, optimum temperature, heat inactivation and Michaelis constants and their inactivation by chemical group-specific reagents. In order to examine the relationship between antigenic variation and enzyme activity of the influenza neuraminidases, three X31K monoclonal variants were selected using anti-neuraminidase monoclonal antibodies. Two of these (X31/NC92 and X31/NC56) were modified at a single neuraminidase epitope, and the third one (X31/NC92/NC56) at two epitopes. The neuraminidase activity of the monoclonal variants was analysed and compared to that of the prototype strains. Compared to A/Hong Kong/1/68, the A/Bangkok/2/79 strain neuraminidase was more susceptible to inactivation by physical (pH, temperature) and chemical agents [urea, dithiothreitol, 1-ethyl-3-(3-dimethylaminopropyl carbodiimide), iodoacetamide, acetic anhydride, 2,3-butanedione] and showed a twofold lower substrate affinity for N-acetylneuraminlactose. The neuraminidase activity of the monoclonal variants of X31K became more susceptible to inactivation by both physical and chemical agents than the original strain and exhibited various substrate affinities. Therefore, we conclude that the enzymic properties of the structurally conserved active sites of the neuraminidase molecule may be influenced by antigenic modifications that affect the variable areas of the neuraminidase and that the degree of this enzymic variation is related to the nature and number of the modified epitope(s). A local conformational change in the neuraminidase molecule reflected as antigenic variation could be involved in modification of enzyme activity.