Abstract
The nucleotide sequences of the haemagglutinin (HA) genes of influenza A (H3N2) isolates from the 1985-6 season in Japan along with those of several viruses isolated between 1982-5 from other countries were analyzed to determine the origin of the 1985-6 Japanese strains. The HA genes of these viruses consisted of 1762 nucleotides and had a three-nucleotide deletion downstream from the stop codon when compared to the sequences of earlier Hong Kong H3N2 viruses. An evolutionary tree of the HA genes of these viruses was drawn using the A/Bangkok/1/79 sequence as the starting point. Eight strains isolated from Asian and Pacific regions including Japan in the 1985-6 season (one in May) had the HA genes located closely on the evolutionary tree but away from those of the isolates in North America and Europe during the 1984-5 season, and a common ancestry for these viruses was suggested.