The nucleoproteins (NP) of a group of influenza A virus strains were analyzed with the use of a panel of anti-NP monoclonal antibodies (Mabs) in the radioimmunosorbtion reaction with subsequent polyacrylamide gel electrophoresis of the immune complexes, and in the immunoblotting. The group included 2 pairs of closely related viruses having minimal amino acid differences in NP sequence. The analysis of the results of the immune reactions in comparison with the known amino acid sequences of NP allowed us to suggest that the Mab 150/4 recognizes the epitope containing amino acid residues in positions 196 and/or 290, whereas the amino acid residue in position 353 participates in the formation of the antigenic epitope reacting with the Mab 7/3.