After 12 passages of a mouse-nonpathogenic influenza A/USSR/90/77 virus in mouse lungs a pathogenic virus was obtained causing death of the animals at 4-7 days after intranasal inoculation. The genetic and structural analysis of the initial and pathogenic viruses performed by oligonucleotide mapping of individual virus genes demonstrated that in the course of adaptation to mice structural changes had occurred at least in 5 out of 8 genes of virus with the exception of the genes coding for matrix and nonstructural proteins. The greatest differences were found in the genes coding for surface glycoproteins: hemagglutinin and neuraminidase. The experimental results indicate an important functional role of surface glycoproteins of influenza virus, particularly hemagglutinin, in the process of adaptation and formation of the pathogenic properties of virus.