Two groups of antigenic recombinants Hav4N1 were obtained by recombination of human influenza virus H0N1 with two avian influenza viruses isolated from ducks in 1956 and terns in 1978 and possessing the same surface antigen Hav4 Nav1. Recombinants obtained by crossing A/PR/8/34 and A/duck/CSSR/56 viruses showed a lower ability to reproduce at optimal and lowered temperatures and differed in the thermosensitivity of haemagglutinin and neuraminidase. An analysis of virus-specific proteins of the recombinants revealed different combinations of genes coding for internal (Pl, NP) and nonstructural (NS1) proteins. Recombinants obtained by crossing A/PR/8/34 and A/tern/Frunze/334/78 viruses possessed a thermostable haemagglutinin; they produced plaques of a size characteristic of avian influenza but, as distinct from the latter, they were practically not eluted from fowl erythrocytes. Polypeptide analysis of these recombinants showed that the genes coding for NP, M and NS1 proteins were inherited from the A/tern/Frunze/334/78 strain.