The North American low pathogenic avian influenza A virus H7N2, that lacks the 220-loop in the hemagglutinin (HA), possesses dual receptor specificity with a strong affinity for the avian and a weak affinity for the human sialic acid receptors. To estimate the possibility of interspecies transmission by H7N2 viruses, we have serially passaged a chicken virus strain, through mouse lungs, to adapt the virus for mice. Once mouse adapted (MA), the original apathogenic virus became virulent with 13 amino acid substitutions in five viral proteins (PB2, HA, NA, NS1, NEP). The four mutations in the HA1 chain did not significantly change receptor specificity, but elevated the HA thermostability and the pH value of HA activation. NS1 was the most modified protein, with six amino acid substitutions. All substitutions occurred at sites that were polymorphous in the original wild strain. The minor alternative amino acids of the original virus NS1 remained in the MA variant. The MA influenza virus also acquired the adaptive mutation E627K in PB2. In addition to E627K in PB2, four other mutations (N73T, G171A, F214L in NS1, and E14Q in NEP) can be considered to be adaptive because they are common among viruses H7N2 isolated from mammals (human, cat, and mouse).