Pigs were intranasally infected with avian influenza A/H5 (H5N1, H5N3) and A/H4 (H4N6, H4N8) viruses in mono- and coinfection. Infection with both apathogenic and pathogenic strains caused no clinical manifestations. A virus and/or fragments of its genome retained in nasopharyngeal fluid as long as 6-8 days after infection. During monoinfection, the structure of the hemagglutinin (HA) receptor site of isolates from the pigs infected with A/H5N1 strains (A/chicken/Kurgan/3/2005, A/duck/Russia/5354-vac/2005) and A/H5N3 (A/duck/Primorje/2633/01) remained unchanged during 6-7 days. When two animals infected with avirulent A/H5N3 viruses (A/duck/Primorje/2633/01, A/duck/Altai/1285/91) that differed in immunogenic properties were kept together, the A/duck/Altai/1285/91 virus that induced a later IgG generation was prevalent in the nasopharyngeal fluid of both animals. Moreover, 4 significant nucleotide replacements were detected in the HA gene on days 7-8. Infection of pigs with avian influenza A/H4 viruses yielded the similar results. The joint keeping of animals infected with Algarganey teal/Astrakhan/309/102 (H4N8) strain and A/ musk beaver/Buryatia/944/00 (H4N6) isolated from musk beaver exhibited fragments of "a variant" of the identical structure in the nasal swabs of both animals on days 7-8. A nucleotide sequence from 37 nucleotide replacements differing from both baseline sequences was revealed in the HA 364-1045 gene region. The amino acid sequence of the variant was similar to Algarganey teal/Astrakhan/3091/02, other than one position 264 < Lys > (numeration by H3), which coincided with the A/ musk beaver/Buryatia/1944/00 strain. The latter induced the antibody generation on day 5 after infection while the A/garganey teal/Astrakhan/3091/02 strain did only on day 14. It is possible that under co-circulation of two different influenza A viruses, the virus causing a slower development of an immune response showed a higher probability of transiting to another host (specimen) and changing the HA receptor site in the organism of a new host.