Influenza remains a serious cause of disease in man and of death in the aged. Vaccination is only transiently effective in providing immunity1, because the surface haemagglutinin molecule of the virus can evolve rapidly, allowing the virus to escape neutralization by immunizing antibody. Of the three influenza A subtypes2 known to cause infection in man, the H1 subtype is of particular interest as it has proved to be the dominant human subtype this century and is now co-circulating with variants of the H3 subtype. Here we present the sequence of the haemagglutinin gene of an early H1 subtype (strain A/PR/8/34) as determined by recombinant DNA methods and dideoxy sequencing. From the deduced amino acid sequence of the haemagglutinin we define an antigenic site at amino acid residue 160. Comparison with haemagglutinin molecules3-9 shows that the H1 and H2 subtypes are more homologous to one another than to the other subtypes, which suggests that they have diverged only recently from one to another.