Wild waterfowl are currently considered the largest reservoir of the various haemagglutinin (H) and neuraminidase (N) subtypes of influenza virus. Until now thirteen different H-types and nine different N-types have been detected in these populations. In the first instance, virus transmission from fowl to other animal species and to man is not causing disease problems. However, small changes at the molecular level of a given HN-subtype recently caused a dramatic increase in virulence for chickens. Genes fragments coding for haemagglutinin or neuraminidase can be exchanged between viruses which propagate in the same individual. This phenomenon-'genetic reassortment'-is of major epidemiological significance when it occurs in pigs. New influenza epidemics in the human population consistently originate in areas where waterfowl, pigs and human beings live close together. At the moment, the virological and serological diagnosis of influenza A infections is based ELISAs for antigen and antibody detection. Both ELISAs employ a monoclonal antibody directed against a conserved antigenic determinant of the influenza A nucleoprotein. The use of these tests can simplify the diagnosis of and screening for influenza A infections, particularly in those species which harbour several H- and N-subtypes.