An antigenic analysis was carried out on 145 duck influenza virus isolates of the H3 haemagglutinin subtype obtained over five years continuous surveillance from the region of southern China, a hypothetical influenza epicentre. This was done using a panel of twelve monoclonal antibodies raised to an early human strain of the H3 subtype. We demonstrate the existence of an extensive range of antigenic profiles, broadly similar but not identical to the human H3 strain, which persisted over the five year period. This variability was as great during discrete twelve month periods as over the whole five years. Hierarchic progression (observed with human strains) was not evident and no correlation of antigenic drift, in either positive or negative direction, was observed with the domestic duck isolates over time. Changing dominant antigenic profiles were, however, observed in faecal isolates with time within a single farm. The much broader range of profiles detected in pond water samples from the same farm suggested the existence of a heterogeneous antigenic reservoir. Local switching of dominant profiles may occur due to changes of cohorts as birds are taken to market. In vitro and in vivo passage experiments revealed a high degree of heterogeneity in antigenic profiles in progeny of uncloned isolates, whereas the profiles of cloned isolates were largely conserved. These results suggested that particular antigenic profiles in primary isolates may result from mixtures of subpopulations of the wild type virus in natural duck infections. Switching between reactivity profiles of different progeny is likely to be largely a result of regrouping of these subpopulations with lesser effects due to mutation. Hypervariability in some of the cloned isolates was observed with a few monoclonal antibodies recognising a region of HA reported to be hypervariable in swine influenza virus. Reactivity with one particular antibody was correlated with passage in chicken eggs. The ability of this enormously varied pool of duck influenza H3 strains to cross the species barrier to man and give rise to viruses with hierarchic capabilities was considered.