The nucleotide sequence of the nonstructural protein NS1 of the influenza virus A/equine 2/Suffolk/89 was determined and found to be 97% identical to that of A/equine 2/Miami/63. A similar level of identity was shown for the deduced NS1 amino acid sequence. The NS1 gene was expressed, in its entirety and in part, as fusion proteins with glutathione S-transferase using, the pGEX-3X expression vector. Antibodies to NS1 protein were detected in serum samples from ponies experimentally infected in influenza virus, but not in animals vaccinated with whole inactivated virus or in unprimed control animals. The antigenic determinants(s) of NS1 protein appear to be located in the C-terminal half of the protein.; The invention makes possible the use of NS1 protein as a diagnostic marker for influenza virus infection in the presence of high levels of circulating antibody to influenza haemagglutinin generated by recent vaccination. Use as a potential vaccine component is also possible. A recombinant fusion protein of glutathione S-transferase and the C-terminal half of NS1 (incorporating amino acid residues 105-230) was tested in an ELISA to detect influenza A virus infection. The detection rate of the ELISA compared to the methods used for the detection of anti-haemagglutinin antibodies was 60-79%. It was able to detect infection against a background of vaccination in samples from experimentally challenged animals and from training yards in which infection following vaccination was suspected.