The growth characteristics of a series of influenza A viruses in the turbinates and lungs of hamsters was measured: in addition, the susceptibility of hamsters to infection by these viruses was also determined. These two criteria were used to give estimates of the growth potential of influenza viruses in hamsters, and the results were related to the incidence of transmission of virus from inoculated hamsters to cage-contacts. The results showed that strains of influenza virus reported as virulent for man tended to grow to higher titres in hamster nasal washings and lungs; were more infective for hamsters when inoculated by the intranasal route; and showed a high incidence of spread to cage-contacts. The methods could provide valuable measurements of virus attenuation and transmissibility for man, and the further exploitation of these techniques could facilitate the production and licensing of live, attenuated influenza virus vaccines.