During the 1996 influenza epidemic in Vienna we obtained influenza A virus specimens (Vienna/47/96, Vienna/81/96) which grow efficiently in African green monkey kidney (Vero) cells but not in embryonated chicken eggs. Amplification of the specimens in Vero cells resulted in progeny that agglutinated human but not chicken erythrocytes. Reassortment analysis suggested that the haemagglutinin (HA) might be responsible for the host restriction. Vero cells were infected with the Vienna/47/96 virus and then transfected with reconstituted ribonucleoprotein complexes containing HA genes from egg-adapted strains. Subsequent selective passages in embryonated chicken eggs resulted in selection of transfectant viruses, growing in eggs and containing the transfected HAs. The results demonstrate that host restriction of the Vero-adapted Vienna/47/96 virus is due to its HA. Moreover, the experiments showed that the Vienna/47/96 strain can be used as helper virus for reverse genetics experiments.