Nucleotide sequence analysis of the region of the haemagglutinin gene coding for the HA1 domain of the protein was performed on 19 human influenza A strains of H1 subtype representative of the two epidemic periods from 1977-1983 and from 1950-1957. The amino acid changes relative to A/USSR/90/77 are summarised and are consistent with the view that variation in these field strains involves changes largely at the Sb and Ca antigenic sites previously characterised in laboratory mutants of the haemagglutinin of influenza A/PR/8/34. The Sa and Cb sites are less variant and are probably masked by carbohydrate side chains. We discuss the significance of other amino acid changes which do not correspond to previously defined antigenic sites. We also define the "mainstream" amino acid changes characteristic of the divergent evolutionary pathways of the 1950-1957 and 1977-1983 periods and note that the rate of evolution is faster in the earlier period.